I think the only way to really get people to show up is to have team captains coordinate with each other and schedule their matches on their own. Having pre-determined times probably isn't the best when you have to get so many people online at once. The Eu duel tournament worked out nicely since you had nearly and entire week to work out your duel time. I think it should be left up to the team captains to talk amongst their members and then talk to the opposing team's captain.
Captains are definitely necessary because as long as someone tells me the day before or sends me a steam message I will likely show up.
