I also wrote some code to convert the SOW master OOBs into divisions for GCM, so we can use historical OOBs for this kind of mini-campaign, for a change.
To see what it's like you can run the GCM Launcher and click Game -> Load Campaign, and select the test game. You won't be able to play it though.
The main idea for rules would be that there are two players who are actually playing the campaign, handling the strategic part through the launcher and leading in battle. Anyone else who wants to be in the battle would offer to help one of the players, and that player would decide what to let him command in the battle, no one would be obligated to take part. Obviously it will depend on these two players to make it work.