I think that to get this kind of thing going, you just need to dive in. Set a date (weekend maybe), make up some rules and contact the few people who promised some prize money.

Personally I believe there needs to be a prize to set the mood. It doesn't need to be big, but it needs to exist. And that doesn't happen as a community project, it will just get the "death by committee" fate. Would be cool to get a semi regular thing going on, something like once a month or two weeks. But before it gets to that level it needs a determined and level-headed organizer, who will get the wheels turning.

So, if you feel like it, go for it! Don't wait for others. If you have a good thing in your hands, the people will come.