Donovan's "Jennifer Juniper"

Question

In the last verse of Donovan's "Jennifer Juniper," which is a French translation of the first verse, I have a question regarding the third line. This line reads:

Jennifer Juniper assise très tranquille

and is meant to be a translation of "Jennifer Juniper sitting very still."

1. The word 'assise' must be part of a truncated grammatical construction made such for the purposes of versification, and I am trying to figure just what that whole construction is.
2. It could not be reflexive, i.e., she sits herself still, because that is "elle s’assoit tranquille."
3. The obvious answer seems to be a Passive Voice construction, but "elle est assise" does not admit a Dative of Agent as all passive constructions must.
4. Therefore, is what I am seeing more like what is called in English an "Agency of Noun"? If true, 'assise' would be rendered as a verbal adjective/ participle and as such required to agree in number/gender with the implied female subject that is Jennifer Juniper and thus is why there is an "e" at the end of the word. I am leaning towards this explanation, because in Latin and their romance derivatives adjectives are often used substantively, but I've never seen that with verbal periphrastic constructions. Still, "she is a sitting (that is) very still" seems awkward.

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Jennifer Juniper lives upon the hill
Jennifer Juniper, sitting very still
Is she sleeping? I don't think so
Is she breathing? Yes, very low
Whatcha doing, Jennifer, my love?
[...]
Jennifer Juniper, Jennifer Juniper, Jennifer Juniper.
Jennifer Juniper vit sur la colline
Jennifer Juniper assise très tranquille
Dort-elle? Je ne crois pas
Respire-t-elle? Oui, mais tout bas
Qu'est-ce que tu fais, Jenny mon amour?
Jennifer Juniper, Jennifer Juniper, Jennifer Juniper
[ Donovan Leitch, Jennifer Juniper lyrics, © Donovan (music) Limited ]

Answer 1

That assise is here a participe passé used as an adjectif (that must be made to agree in number and gender), to denote that she is assise (obviously). I am not sure that bringing in "agency of noun" is useful here. It's more about a participe passé used as a mere adjectif.

One is reminded of the famous (from Le Corbeau et Le Renard, Jean de La Fontaine, Fables, 1688):

Maître Corbeau, sur un arbre perché,

where perché seems to have the exact same role as assise in your question.

I would also add, that in Jennifer Juniper, sitting very still Is she sleeping? there is something more going on, which is that we are wondering if she is sleeping precisely because she is so still in her sitting. In that case, I might translate as: Jennifer Juniper, si tranquillement assise, est-elle endormie? either bringing the tranquillement forward for emphasis, or at least introducing si to connote that it's precisely because she is so still that we are wondering if she is sleeping.