What about cities that rebel from rebel civs? If you had some English colonies rebel, you might have a civ called Rebel English (America). If some cities rebelled from the rebel empire, would you have a civ out there called Rebel Rebel English (Confederacy)? And what about the second rebel civ from the same origin? Would it be Second Rebel English (South Africa)?
I think a list of rebel civ names would be better, similar to the city lists. Rebel nomenclature should be reserved for when the list runs out. Other than that, it would be absolutely fun and is plausibly justifiable. However, invisible stuff you can't do anything about is never THAT fun, so as long as you are doing this, why not put in a capability, say a Building called Polling Institution, that reveals rebel culture.