If you get the Godslayer, he cannot equip any items - he is stuck with the demon parts config that the AI gave him. I realize that activating the parts management mechanic might be hard, since it's a faction mechanic, but if thats not possible then maybe it would be possible to just allow him to equip stuff like a normal demon prince?

That would be pretty fluffy too, since after some campaigning his form would probably stabilize and he would not be able to mutate freely anymore.