We have a regular group of four and I have started GM'ing. So here's what we figured-out:

Next time we play a campaign, all four of us will roll a new char (I'll be the GM but will also login with a second license and create a player char).

Once all four chars are created, the GM will reassign their char to one of the other players.

We play the campaign, and save at the end.

Then, we zip up the save game and send it to the next person to be GM. That GM adds content to the save game file to create their campaign.

To start the next campaign, the GM simply loads the save, invites the players, and assign the pre-rolled characters.

It's not elegant, but we tested it and it seems to work fine.