IMHO, the Spring spell should put out pyre zombie flames, or at least have a chance to do it. Doused pyre zombies would become, oh I dunno, diseased corpses maybe?
Also, undead could be like golems and not heal. Perhaps then they could only heal if they actually parked right on top of a death mana site. Could make for some interesting hordes of undead hanging around the Broken Sepulcher and other death nodes.