OK, sorry, I'm so dumb... You can't call it with a script portion, you actually have to call up the FOSSIL add-on module, and then, set OpenCharacterCreator 1 there...
Fantastic, now I can create a character. But when I validate, the game crashes and I get
TypeError Cannot read property '0' of undefined
Answer : Actually have to put the character in the team BEFORE calling the script to edit it.
Now, it works like a charm ! Thank you sooooooooooooooo much !