Ace Races should not sell out.
Worst case, you have a crowd of people with 2 discs in their possession.
Then you have a few people who didn't sign up. To which you politely ask your crowd: "Would anyone like to let someone borrow or buy one of their discs?"
The trick is telling the non-signed up crowd to come back after you get all your pre-regged people their stuff.
It's a beautiful thing seeing people cooperate and "make it work".
No one gets turned away at our Ace Race. It just might be a tiny bit awkward for a minute or two, but it's worth it to let a handful of people enjoy the day with you rather than telling them "better luck next time."
This used to be a legitimate solution to accommodating walk-ups when you don't have enough packs; however, with Discraft's rewording of the rule that "every player must buy a new full pack for $25 in order to be eligible for the grand prize," TDs should be careful in offering this option. In previous years I did just what you outlined above, and even had some couples play off a shared pack (each gets one disc and they can fight over the rest of the pack contents) by just adding a player to their pack for $10.
Of course, allowing those walk-ups would be the cool thing to do, but if word gets back to Discraft from a cranky player (especially if one of your "add-on" players ends up winning) that you broke the rules, your future events could be denied...my event was almost refused this year -- even after it had been scheduled -- because I was advertising that people could play for $10 if they had discs from a prior 2013 Ace Race (which is something else I've done for years).