Well, this idea’s been batted around a few times, always met with “No way, players would totally abuse that”, and those critics of this idea are absolutely right. So, we just need to outsmart the abusers.
So how about we simply make it so the custom names of thralls are only viewable by oneself and fellow clan-mates? That way it’s easier for you and your friends to tell your thralls apart, mark your favorites, and you don’t inadvertently break immersion with other players who probably wouldn’t be quite as amused to see your castle being guarded by the Spice Girls. To the random passersby, it’s just another exile on guard duty.