Unless there is another definition of a "hallowed area" that I'm unaware of, an area under the effect of the hallow spell would qualify. It's fairly clear that undead CAN enter a hallowed area by the definition of the spell. Why would it grant a bonus to turn creatures that couldn't enter the area? Also, there is nothing that specifically forbids them from entering such areas in the spell description.
So they could heal vile damage in a hallowed area...it just isn't likely to be available to an undead creature. If you are talking about a lich though, they could easily disguise self and pray in a church until the damage was healed.