This is a very interesting topic. I think there are ways to do a multiplayer campaign.
I have not tried these myself, but I thought I should mention them here.
#1 fwatchAs far as I understand anything, Kegetys' fwatch program, which can be found from this thread:
http://www.flashpoint1985.com/cgi-bin/ikonboard311/ikonboard.cgi?s=8d47448a66ee23c5d284cea817dd3a51;act=ST;f=7;t=46245;st=0allowes saving variables to an external file and of course loading them back also. In other words, you could save variables to a file during a multiplayer mission and then in the next mission you could load them back and thus adjust the mission like it were a campaign. The problem with fwatch is that currently it only works on Windows so it is not a general solution
yet.
#2 Sinews of WarThen there is Sinews of War:
http://www.sinewsofwar.com/home.phpSinews of War
seems to be a working solution for all servers but it is somehow limited. If we get a version of fwatch which works on both Windows and Linux servers, fwatch would with no doubt be a much better solution than Sinews of War.
I have not tried either of these myself (except fwatch's demo mission) but please go ahead and do research on them if you have the time to do it. Please post your experiences here if you decide to get your hands dirty.