That would still make signing up online necessary, but won't punish people that forgot/didn't know about that'/randomly decided to go.
The second group is a valid concern, but that can be offset with strong promotion.
The first and third groups need to be sharply discouraged for events of this size. Many who would have no
real problems signing up somehow imagine doing so as the most difficult thing in the world, and while they
do plan on going, they can't be bothered to let the TO's know ahead of time if it isn't required of them. TO's have to work very hard to accommodate behavior like this when they could be focusing on improving the event and resolving potential hiccups like, say... checking with the fire department.
Capping signups at the door might a good idea if the TO's feel the need to quell whining, but I imagine it would invariably produce more whining about much more upsetting problems. Imagine if someone high-profile "forgets to" (i.e is too lazy to) sign up, but drives/flies a significant distance to come anyway. If door signups are already maxed out, there will be whining about that, and likely our subject will guilt-trip and community-appeal their way into the tournament. Then, other people become upset because it makes the registration rules and caps seem meaningless, and they were *such a burden* weren't they? This
would happen. Guaranteed.
The real problem is never the signup rules, though. It's that some smashers don't like to be faced with responsibility of any kind, and would rather someone else have to work very hard than to have to make an adult decision. This particular problem should not be allowed to hold back the rest of the community.