Here's an odd one. I'm searching for bands/albums that utilize choir, Gregorian chant and the like. Preferably black, death, thrash and doom, but I suppose that's being a little too optimistic.
Deathevokation uses a choir to introduce "As My Soul Gazes Skywards," which is a very epic and very good song off a very epic and very good album. Probably not precisely what you're looking for, but I recommend checking it out regardless.
I have no idea why I didn't think of this earlier.
I know you said black/death/thrash/doom, but this doesn't really fit any of those. It's just slow, epic, symphonic metal. All the vocals are done by choirs.