Yeah, I'm always amazed how quickly events sell out. I think Catalyst could offer twice the number of events and they would still sell out. But Bull would have to perform at least a few "friendly extractions" to find enough GMs to run all those tables...or find a way to plug in existing GMs to do nothing else the entire con.
You might check the Gen Con registration site from time to time during the prereg period (until like June 20, I think). This year, they're allowing event ticket returns online, so you might find a couple of tickets as people's schedules align. And there's the time-honored tradition of showing up with generics. The table sells for 6, but most GMs will take a 7th player, and there's always the chance that someone doesn't show (or oversleeps the 0800 slot). I think last year's policy was that unanimous approval of the table was required to take an 8th player, since an 8-player table tends to take longer and allow less individual character interaction. The generic route is a crapshoot, but if it's the only option...
You could also try asking a GM to run a game at an empty table, assuming you can find a spare GM, spare table, and have/accept enough players to do it. You may have to pay generics as if the event were scheduled; I'm not entirely sure how the con would want that handled in an allocated space. Check with the room captain at the con if you want to try this, or maybe Bull has some insight on this. And I'd try to be flexible with the particular mission for this, since the GM you're grabbing may only have a couple prepped. It's a little more legwork, but it might be worth a shot.