Just what it says on the tin; use the special mobs feature to enable the spawning of charged creepers (since potion effect creepers cause trouble with unlimited duration lingering effects).

Hmm, since I fixed the lingering effects a few versions ago I might pass on this. It's a great idea, but since the vanilla method of getting skulls is a delicate balance, I'd hate to really play with that too much. I'll save this idea for later for sure, I like its potential!