I'm okay with that restriction. I don't like the idea of restricting people with X amount of posts. I think it's at least worth a shot. We've never really had anything like this before, and I think testing the waters (maybe have a trial period where supporters can access it to see how it works and how well it holds up?) a bit could help. Worse case scenario is it doesn't turn out to be what I'm hoping (and expecting) it will be.
I mocked up a forum: