Windworx is developing the project Warsoc, which aims to improve the multiplayer functionality in multiplayer games (more than 12 players and the possibility to join to a game even when it has been started). With the technique he's developing, you could do a campaign to be played in multiplayer in theory.
Anyways it's in developing process and I don't know if this can be achievable.
For more info, check here:
http://hiveworkshop.com/forums/showthread.php?t=35229