That is a very interesting idea. In order to do in on this website, though, it would require the sharing of passwords, which is probably not feasible with 21 people. Alternatively, we could simply create 7 new accounts ONLY for such a tournament, with three people possessing the password for each account.

Just as an idea (though this is probably too complicated) - here's how I was introduced to the game of Diplomacy some 15 years ago :)

1. We had an International Relations class at university and the professor divided us into 7 teams, 7 people each.

2. One from each team was elected 'President', the others were assigned as ambassadors to the other 6 nations. Effectively, everyone got a counterpart with whom to talk.

3. At the end of each round the teams would gather and discuss. All ambassadors came with specific move suggestion based on their meetings. I cannot remember how exactly was the voting done - but the President was the key as he had more votes (maybe 3, while everyone else had 1).

Anyway, my point was that it turned out to be a great game as:
1. The level of communication was exceptional
2. There were always many alternatives and every idea was defended by someone (the respective ambassador)
3. The decision-making process (e.g. what to move in the end) was a lot of fun.

Well, some teams did not do very well and started blaming each-other - but you always get this in a team game :)

Oh, I forgot - I like the idea in general - so if you decide to do it and need players I'd be happy to join :)

I'd want in on such a game.