Van Morrison isn't obscure or anything of course, but I think his work with the Chieftains is kinda obscure- which is a shame, because their collaboration produced some gems, like this one, Star of The County Down:
Ah, how about some more first generation Manhattan Transfer? This is from their first album circa 1969 - a beautiful rendition of "Fair and Tender Ladies" - its certainly old and pretty obscure to boot now: Fair And Tender Ladies
I saw this guy at Folklife, a big festival in Seattle held annually in May. It was some time in the late 90s. I don't think he ever really got discovered. So it's definitely obscure and starting to get a bit old too?
Here's another (non-native) Nutmegger. When David Geffen first heard Laura Nyro, he dropped everything to manage her. Unfortunately almost all of the footage of her at Monterey was lost, and never got much traction as a performer. I like most of her performances of her songs, multi-tracked vocals (which I don't normally like) and all, better than the more famous versions. Here's one of my faves: