Apologies in advance for the 4 year thread necro, but I've gone back to the Charlemagne scenario recently, and I was wondering the same thing. My initial thought was to do as exactly as suggested here, that is, change all PlayableCiv values to 1. But that didn't work.
Has anyone fiddled around with this scenario to make Rome, Byzantium, etc. playable? There must be something somewhere in Assets that overrides the PlayableCiv values in the WB save file.
Of course something tells me that trying to play as Rome would break something...