I don't know if the Breton had legends of lands across the seas like the other Celts, but that could be a potential source for names.
Hmm, now that you mention it, there is mention in Irish mythology about an island called "Hy-Brasil".
https://en.wikipedia.org/wiki/Brasil_(mythical_island)
So in that case probably using a variation of a theme in this case? Granted, judging from the description on Wikipedia (which has more than a strong shade of the backdrop for the musical/film
Brigadoon) it sounds like a description of Newfoundland and in particular the Grand Banks (which are foggy to begin with because that's where the Gulf Stream meets the Labrador Current, and thus respectively warm and cold air continuously interact), but using the myth to describe the whole area of New France, or at least the area of Canada, Acadia, and Plaisance (aka Newfoundland) in one go? We even have an actual French form as well as gender ([le] Brésil), unless one decides to shift gender to disambiguate between the two Brazils by making the North American Brazil take on the same gender for <île>, the word for "island" - in which case the actual mythological concept can be used in shortened form, from "l'Île de Brasil" to simply "la Brasil".