Does your location in Maine mean you are limited to North America in your search, or do you sometimes travel to other countries? I'm thinking of the slate quarry at Blaenau Ffestiniog in Morth Wales, Llechwedd Slate Cavern I believe it's called, which you can visit by getting on a tram and descending "down in the dark, deep underground," to borrow a phrase. Perhaps after the tourists went home for the evening you could arrange to stay late... I don't know what the acoustics would be like, but I like the idea of it, especially because in times past the quarrymen were known for their lunchtime gatherings that were like a mini-Eisteddfod--competitions in singing, recitations, bible quoting, extemporaneous speech-making, etc. So there is a history of the space being used artistically. Margaret