Could [games with too-few players] be canceled earlier, so players could get reassigned before the Con?
With our new system of scheduling, we should be able to cancel games that we know are not full. No members will have a "blank" schedule or blank slots in their schedules, and so there will not be "pick-up" players wandering around hoping for a game.
We have also changed the form to encourage GMs to let us know their game choices in case their game does get cancelled, so that everyone involved can simply be moved to another event they would enjoy.