Here's an odd thought I had just after waking up, thinking I've overslept:
I've heard that people have to change over their monitor plug from their main (cruddy) vid card to their xxx (better) card, flip the menu bar over in the Monitors CP, and then restart to have the monitor recognized. Just when they want to play a game.
What about this:
•A Dr. Bott Moniswitch, or similar
•on the end of one of the switchbox settings, connect one of those adapters that forces a resolution. IIRC, this makes the Mac think there's a monitor connected
Now you have the single monitor hooked up to the switchbox, and when you need to play a game, do the menu bar setting, and switch the video to the appropriate card.
Disclaimer: This is just an idea I'm proposing. You are free to use it, but if you muck up your own hardware, I can't be held responsible for what you've done
Please feel free to add/flame/tell me this has been posted before and I'm not the first