I think that sounds a little too bureaucratic. Like Wanderer said, this is more of a casual gig...
One thing that might work would be to tell would-be riffers to find a couple of other newbies (or kind newbie-friendly souls) and a message board of their own, and get wacky on a short 'fic together before they start MiSTing stuff over here. That way, the newbies get to show off by riffing stuff and writing sketches, the rate at which newbies filter in will be considerably slower, and everyone (theoretically, at least) should end up satisfied.