Isabeau is feminine, an older French variant of Isabel/Isabelle. Belle and Beau are incidental, not really the connection, it's a diminutive form, -eau being a standard diminutive formation of the time.
Don't know if they count as 'historical' but Isabeau is a family name of my father's. Marie Esabeau (a variant spelling, you can see the connections between Isabel and Elisabeth very neatly there I think) Colin b. 1692, granddaughter of Esabeau Carrel b. 1640 or so, both of Neuchatel, Switzerland. And there's Isabeau de Bavière (1300's) and the libretto Isabeau by Mascagni.
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