I had hoped to be able to rig up movement in Final Fantasy type games to mouse input but it doesn't seem possible. Does anyone with PC game experience know if it's possible to map the walk keys (of FPS games etc) to mouse input? My son plays FPS stuff a lot more than me!!
It really depends on the game. Some games are configurable up the whazzooo. Other games are a little more restrictive in the configuration options. Best thing to do is to check the game itself out and try and configure it.
Failing that, if you're going so far as to construct an omni directional treadmill and deal with the engineering that goes behind it, creating a standardized interface seems almost trivial. A multi purpose interface seems to your best bet. Default to a keyboard (pulsed or continuous, whatever you see fit) and fall back to a mouse or trackball based interface. One thing I would take care is to standardize the interface, so all the PC sees is a keyboard or mouse. With the USB interface, you can probably create various modes and invoke them whenever you see fit. This minimizes interfacing headaches with the games in question.
In any case, omni directional treadmills look awesome. I didn't realize how far along the technology came along in the last ten years.