No, no, and no. Definitely not. The only way this should be implemented is in 1 v 1 games.
Reasoning: Concede button cheapens the game.
How many times have you all heard some new player complain they're position is gone and they're dead when with a little diplomacy, they can resurrect their position? All the Concede option is doing is giving those player a chance to say "screww it". We should be encouraging playing until the end, not giving up. The best games are the ones where people dsuke it out until the end.
@PE - why should a concession be better than a defeat? If you're conceding to a 2wd, you theoretically would be eliminated. Simply put, conceding IS defeat. A concede button says "I give up", which means defeat. On principal, I would never accept a concede vote if it meant they get a survive instead of defeat. Finally, part of the game of Diplomacy is bluffing. If you're willing to take the risk of a 2wd, you should have to earn it all the way to 17-17 and risk the stab for a solo. Otherwise, don't go for a 2wd. Giving a cheap way out by everyone conceding when the 2 allies are at 13-14 elminates part of the inherent risk of a 2wd.