Well, I just answered my question with a few minutes research. The Byzantines had soap. Looks like for the most part it was introduced by the Muslims, around the 600's or so??
Anyway, there seem to be references to Byzantine soap makers and soap sellers... so my guess is that aspect of the Baths in Byzantium would have been different after the 7th century.
Sounds like some enterprising Citizen might be able to make money making and selling "Byzantine Soap"... :) I'd buy some I think!