It's a difficult question and opinions are mixed, as far as I can tell. I would lean towards the singular, but several decent resources recommend either an option or the plural.
Try the ear test
First of all, always remember to try your ear with a sentence where the difference is audible. Which is preferable between these?
Ces pots sont chacun très artisanal selon cet expert.
Ces pots sont chacun très artisanaux selon cet expert.
To my own ear the first is markedly better, but then I'm not a native speaker.
Hard to find references that perfectly match our examples
I recognize that an adjective, as in these above sentences, is not quite a past participle, but it's hard to find past participles for which the singular/plural distinction is audible.
In fact, as we look at our various resources, we'll have to extrapolate from other signs of agreement if we want to cobble enough data together, because it seems that agreement on past participles is rarely commented on. Instead, this is the basic question I found discussed in most of the sources:
Ils ont chacun son préféré.
Supports singular agreement as in définie.
Ils ont chacun leur(s) préféré(s).
Supports plural agreement as in définies. N.B. I'm not concerned about the plural on the noun.
A quick eye test as well
The eye isn't quite as reliable, but what do you think of this case, intended to shock one way or the other:
Les courtisans sont chacun allé au festin.
Les courtisans sont chacun allés au festin.
I side with the plural here. (But a similar disclaimer applies: The logic of the agreement in this sentence is very close but not exactly identical to the passive « Elles sont chacune définies par... »)
Grammar: status as a pronoun
Let's think about the grammar for a second to get some background. In terms of its syntactic role, my instinct suggests that we could analyze chacun as either a pronoun or an adverb in this kind of sentence.
The relevance is that if it weren't a pronoun, it couldn't affect agreement so we could rule out the singular.
In English it appears to be an adverb; among various examples I could cite, consider:
The hikers each eat their sandiwch(es).
The hikers each eats his sandwich(es).
Ungrammatical and nonsensical.
However, in French, consensus is that it's a pronoun. Even though it appears in the sentence in a slot normally filled by adverbs, there are other clearly pronominal phrases that can go there too (e.g. « Ils sont tous les deux français. »).
One very good reason to analyze it as a pronoun is that adverbs are invariable, but chacun alternates with chacune.
It's worth noting that another, unambiguously pronominal usage exists in French:
Chacun est artisanal.
I bring this example up because most of the resources first deal with this case, which we have to read past for our own purposes.
Since it is a pronoun, the problem becomes tougher. Then we have two noun phrases: Ces propriétés and chacune. Which one will govern the agreement of définie(s) ?
On the one hand, the plural propriétés clearly governs the finite verb sont. But on the other hand, those two are adjacent whereas chacune then comes between them and définie(s), so could it "seize the reins"?
Or could the speaker maybe choose who gets the reins depending on their intended meaning? (I'll call this varying them based on the "semantic value".)
The problem is that one grammatical reading or the other doesn't seem inevitable to me. That's what we're going to try and guess from our resources.
Credible authorities: no direct mention of this issue
Okay, on to various resources.
French Stack Exchange
Here's another question on this site about this. Unfortunately, it's inconclusive. The answer with one upvote at time of writing suggests that it's the semantic value that determines it.
Specifically, the author says it depends on whether the subject possesses singularly, or all together as one. For our case, then, we'd have to ask if these properties were defined singularly, or all together.
To be honest, I don't think this is a very compelling analysis. The presence of chacun already insists on the individuality, so why would the "all together" sense enter into it?
Here's a brief article I found on "chaque/chacun". Unfortunately, their example sentences raise different issues that are too far removed from our examples to be of any use.
Office québécois de la langue française
Third, this article on the OQLF. Unfortunately, it's not directly useful to us either, though it has good notes on conjugation in the presence of chacun.
Le Bon Usage
... To be consulted someday when I own a copy. :s
Internet grammar websites
Falling back on Google, here are a few resources I found by searching "chacun accord".
The first reference is this article:
Quand le mot CHACUN renvoie à un sujet pluriel et qu'il est suivi d'un complément du verbe, on utilise les formes NOTRE, VOTRE ou LEUR pour le singulier et les formes NOS, VOS, LEURS pour le pluriel.
Nous faisons chacun notre lit.
Ils nettoient chacun leur chambre.
The author then states this caveat for variation on the semantic basis of whether the complement has a "personal" link with the subject:
Quand le complément du verbe a un lien peu personnel avec le sujet, on peut employer SON, SA, ou SES ou bien les formes précédentes. - Cependant, les tournures qui sont en rapport avec le sujet du verbe semblent plus logiques.
Nous irons chacun de son côté / chacun de notre côté.
So this author thinks chacun only takes over with singular agreement if the element being agreed is not personally related to the subject of the verb. I don't see a great deal of sense in this.
Bréviaire d'orthographe française
This article, meanwhile, agrees that the choice is optional, citing again a semantic basis, but this time the one we saw in the other thread here: that it depends on whether you mean they each or they all:
Quand « chacun » est placé après un verbe employé à la 3e personne du pluriel, on peut mettre indifféremment l'adjectif possessif au singulier ou au pluriel, selon que la pensée renvoie à un ou plusieurs possesseurs :
- ils vont chacun de son (ou de leur) côté...
As I noted above, I don't find this too plausible given that the use of chacun excludes the second case.
The author also mentions the indirect object, where that semantic distinction makes more sense:
- ils font chacun ce qui lui (ou leur) plaît...
Does each one do what pleases himself, or what pleases everyone?
However, I think this is far enough from the problem we're trying to solve that it doesn't help us; in définie(s), the relation between the meaning and the agreement isn't nearly so obvious.
This resource has some nice citations under "observations diverses". There's too much here to quote it all, but in summary, the author proposes:
chacun takes son, sa, ses between a verb and a complément direct; between an intransitive verb and a complément circonstanciel; or when the "proposition preceding chacun presents a complete meaning".
chacun takes leur and leurs when not between a verb and a complément direct; when between a verb and a complément indirect; or when "the complément that follows chacun is indispensable to the meaning of the verb".
If we apply these rules to your sentence, we might say that chacun is between the finite verb sont and the complément indirect par une phrase, hence it would take leur; we extrapolate that this means agreement is governed by the plural subject, so we'd pick définies.
That said, although there are citations for each of the above cases, it doesn't seem to me to be enough data to generate such oddly specific rules as those.
This resource strongly recommends a singular personal pronoun or possessive:
Lorsque chacun renvoie à un nom pluriel, il y a parfois hésitation sur l'accord de la phrase (pronoms personnels et possessifs), sans différence de sens. On parle alors d'accord d'intention. Il semble pourtant préférable d'opter pour le singulier (exigé par l'Académie), plus « logique »
Despite this, though, the author allows for the alternative in every example (perhaps because of this nebulous "intention"):
→ Ces artistes, chacun dans son domaine (ou chacun dans leur domaine).
But at the end, the author makes this surprising and obviously relevant claim:
Mais l'accord des adjectifs et participes passés se fait généralement avec le nom pluriel, pas avec chacun.
→ Des articles, écrits chacun par des spécialistes différents
This is a pretty direct answer to your question. Note, though, that the author qualifies this dictum with "généralement" and doesn't attribute it to the Académie or call it more logical.
Two resources that side either with plural agreement or optional based on meaning, and two that would judge this case to be unambiguously plural.
This leaves the arguments for the singular something like this:
When there's an audible difference, I have a preference for the singular (and so does @jcm69 in the comments).
Chacun as a pronoun certainly allows us to believe that it could govern what comes after the verb.
The resources siding with the plural are not high authorities.
Meanwhile, the arguments for the plural:
At least one other person intuits that the plural is correct in another answer. (Also, in the eye test, I had a slight preference for the plural.)
All the resources that directly deal with this question or indirectly through possessives lead us to believe the agreement is either optional or plural.
The verb is evidently governed by the plural noun phrase, not by chacun, as can be seen by its conjugation.
Between these, I would lean towards the singular as having the stronger arguments, but there seems to be a case for the plural.