You could always have a rule where if you've heard the riddle before and know the answer already, you have to wait until everyone else gives up. It wouldn't be enforceable, but hopefully people won't be jerks. I think this is a cool and unique idea for a panel. If you looked online, I bet you could find a pretty large number of riddles to use. And not just regular riddles, but other types of puzzles and brain-teasers as well. It sounds like a lot of fun!