One thing I do if I want to start with a party of 4 in SP is to boot up the game 4 times.
So long as you run it from the .exe rather than through Steam, you can set the game up as player 1, then join player 1's game via LAN. The additional instances show up as debug accounts.
Now although you wouldn't want to play it like this, it means you can completely customise your characters. Once you're through char select, kill the other instances of the game and voila, only one person running 4 characters.
I haven't tested this in GM mode, but I get the impression it would work the same way. You could probably load your custom campaign then have your friend join. Then, have the other "players" join in 3 additional instances of the game. If player formations still work like they do in SP, you should find you can assign them all to your friend.
Worth a try at least
