Q1: When I use the word "chaud" like this in the sense of "avoir envie de", I always couple it with the preposition "pour" instead of "de". I wonder if both are acceptable?
The real idiom is "chaud pour" and not "chaud de". However "chaud de" is used from time to time, as well as only "chaud":
T'es chaud de faire un bowling?
T'es chaud on fait un bowling?
Which are the same as:
T'es chaud pour faire un bowling?
Please note that "chaud de" and "chaud" are less correct than the original idiom "chaud pour". These are oral deformations, you should avoid saying them.
Q2: On Wiktionnaire, the "chaud" in this specific sense is tagged as mélioratif. Does this "chaud" usage come across as more polite than saying, for instance, "pour ceux qui voudraient tester le concept"?
Absolutely not! "Chaud" is also labeled "familier". Never use this idiom when you're not talking to someone you're close to (friend, family...). It is "mélioratif" because it encourages the person you're talking too to do what you are suggesting. It adds a positive feeling to your sentence. As an example, this sentence:
T'es chaud pour un bowling ?
Has a feeling close to:
[FR] Allez viens on va au bowling !
[EN] Let's go bowling!
And less close to:
[FR] Tu voudrais aller au bowling ?
[EN] Would you want to go bowling?