A really interesting idea. MySpace can always get in on the debates because they are a News Corp company. There is also precedent for MySpace hosting debates with the Democratic and Republican candidates.
Just think if everyone on MySpace agreed to boycott and vocally protest (in the real world) any MySpace sponsored debate that didn't include third party candidates. What you might end up with is a MySpace debate with only third party candidates, but that's doubtful. MySpace represents too huge a growing constituency for the major parties candidates to ignore.