At Sidmouth Festival, a few years back, they ran an event called the "Song Slam Competition". It was like an open mike and the winner got a slot at the festival.

Performers were only allowed TWO MINUTES on stage, after which a large hook appeared from stage left and dragged them off.

I've never actually seen the format repeated, but it did have the advantage that the audience was kept engaged. Open mike events at festivals are a bit pot luck, and if the person before you has managed to clear the venue, it's one man and his dog again.