Enabling transferring duchie vassals/granting independence?

I like to roleplay playthroughs, and to be able to pick a small council from all sorts of different houses, so often I'll transfer a vassal over to the Iron Throne for the duration of their Small Council tenure.  But it's not possible to transfer duchies, and while you used to be able to trigger a war for independence to move up to the iron throne, that's no longer an option because of de jure things.  Is there a way to turn that off in options that I'm overlooking?