Okay, I may have missed something or other in the discussion(I eventually started skimming posts), but if you really want to charge panelists reg fees to be returned to them if they do their panel, then I think the most logical way of handling it would be to refund the money after their panel is over.
I mean, even if you refund it if they show up to pre-reg, that doesn't mean they'll still do their panel. They'll just get in and get their money back, and bingo: free entry.
So if you're super cereal about getting panelists to pay, cash back after the curtains. That's the only way