I believe everything has been said here about "Isabeau". The only thing I can add is that this has definitely not been a popular name in the past decades in the French speaking world. Two main reasons for this: - In French "beau" is masculine and that makes this female name sound a bit odd compared to Isabelle - Isabeau somehow survived in the countryside a long time after the original Isabeau disapeared and it became a "country folk" girl name. Illiterate peasants, suspected of not being able to tell the difference between masculine "beau" and feminine "belle", used it to name baby girls. That "hillbilly" reputation stuck to the name and prevented it to become popular again.
This, however, could change, for "Isabeau" lately reappeared among other very "classic" names in France and is now used again, almost 600 years after Isabeau de Bavière's death. Between 15 and 20 baby girls have been named Isabeau each year since 1999 in France, compared to 0 the previous decades (with the exception of a handful of Isabeau in the sixties).
One bit of trivia: Michelle Pfeiffer as LadyHawk bore that name in the movie of the same name.