[thejeff]

You can communion. Or more accurately, if you can communion it will raise your Holy level. A Crystal Matrix will work.

Those with astral magic can also boost holy with PotS.

All that said, I'd expect on a game balance level it's hard to boost because there's less need to. Nations are divided/balanced partly based on how strong the priesthood is, so making it easy to boost would weaken those differentiations.

Also there is less need for boosters. No ritual magic, a small range of spells that don't have a fatigue cost anyway.
Plus there is no gem equivalent to forge items with, so you'd have to balance any such item around another path of magic.