If you have any leftover space at the map, have player 6 own a command center with a couple of workers at a hidden-away spot the player will never find (as long as he's using cheat codes
). Give him some stockpiles of nova and ore by the site.
That should prevent the player from resigning. Oh, and if he is not building a "visible" base, you'll find it a good idea to disable all his or her units (preventing him from creating them) or he may end up building an airbase and sending fighters to the rest of the map