This is pressing so many of my buttons that I can barely type! The Scottish one I mention in my earlier post is where everyone stays in time but sings their own notes and scales. It’s tremendous, I first heard it on that Channel 4 series Derek Bailey did on Improvisation (it’s also mentioned in the book but they had wonderful footage of old Scottish islanders in heavy wool jumpers that really sold it to me).there's also a style called lining out where everyone starts singing the same line but takes as long as they want. when the last note has sounded, everyone moves on to the next line. pure, unadulterated melisma.
I can see this being a new money sink.