I tried something similar back in the day, when all was LWS (I haven't made it all the way through the rules yet, so I don't know if I'm not allowed to mention that), and I had a lot of trouble just getting people motivated. It was pretty much a contest, where I would assign each participant a number, teams would be chosen at random and those teams wrote and recorded one song, and the members of the forum voted on the best song. I got a few people into it, but not enough. If you can get enough people to do it, I'd definitely be up for it.