This is an interesting idea, but there’s an unfortunate paradox. If you’re getting paid nothing at all, volunteering can be a bit of a drag but you can still justify (to yourself) spending kind of a lot of time on it. But if you’re getting paid a few hundred (or a few dozen) dollars a month for spending hours every week, or even every day, on some pretty rough stuff, you’re going to start questioning your earning priorities and look for something that pays two or three times as much, like a job at McDonald’s. There’s a lot of research on how adding financial incentives, however small, can destroy internal motivation.
So I don’t think it’s a good idea to bring money into it unless we’re sure there’d be enough to make it worthwhile.