The Democrats had a debate hosted by the Daily Kos in 2008.
Being as it's a lefty website, the Daily Kos wasn't threatening to run a 3rd party bid for President. It is just plain silly for Republican candidates to attend a debate with a moderator who is actively discussing running against them. Additionally, the setup smacks too much of his realityTV show where he judges their performance and picks a winner (his endorsement). It's just a bad idea and most Republican candidates are going to avoid it. I wouldn't be surprised if the thing is just canceled for lack of participation.