I imagine there are some of you out there who will be interested in this information. We have determined how to build a dongle that plugs into the competition port and allows you to enable/disable autonomous mode.
The circuit above will give you a switch that has 3 positions: Robot Disabled, Normal, and Autonomous mode.
Disclaimer: you probably shouldn't build this. It could void the warranty on your control system. If you don't do it right it probably will ruin your OI. Please don't try it if you don't know exactly what you're doing.