Have some sort of place people can go to check in. After that they get like an achievement for going to the event and maybe access to some special flairs (if subscribed).
Could even try to get a (gasp :O) Smash Ladder setup going at places. Even just with friendlies. Stream it on the official Twitch.

Brawl
Kirby Fighters 2
Melee
Project+
Rock Paper Scissors
Sm4sh 3DS
Sm4sh Wii U
Smash 64
Smash Crusade
SSF2
Ultimate