Nationals has league & tournaments all day, every day. If you want them to run a tournament at any time, just tell the person in charge of the side events (normally John Lathem does them if memory serves me).
I am sure the 1st VG tournament that will be ran will get a HUGE amount of people.
As for VGC rules without registering for the "Battle event" option, you go to the union room and talk to your opponent > Battle > Battles for two > Flat battle/Wonder launcher off. This will have the same ruleset as VGC (minus the whole Unova only clause)