I had an interesting one today: vampires can endlessly gain piety with Dracul and Binlor by repeatedly killing Ying/Yang. Due to lifesteal, you will never actually run out of hit points, so you can just keep fighting without end. Normally killing no-XP enemies grants no piety, but I noticed that Binlor actually grants 5 for killing Ying/Yang, while Dracul would grant piety for life-steal and bloodpool drinking. I'd imagine you could probably work in BLUDTUPOWA and also farm Mystera in this way, and if you had a poison attack you could farm Tikki Tooki too.
Fortunately this cannot be chained with the Pactmaker's Warrior's Pact for infinite health (I haven't checked, but as non-XP granting enemies Ying/Yang shouldn't trigger it) but it's still an easy way to get 100 piety with this class from this subdungeon.