Not trying to crap on your parade, but 500 players is a lot, where do you envision getting those from on such a short notice? Even if you send a message to each cRPG account, send a message to each forum account and invite every Kickstarter backer for a one time sneak peak, you would probably be short. Though 200 people would probably carry the point across too.
Sadly I can't be there, but hope it will be good.