That's something I've longed for, like a lightgun game where you still run around - but really it would need a lot of special coding so it is unlikely that we'd see that in FPSC. The thing is that there's no standard - if M$ made a standard controller with a lightgun, then it could possibly be supported, but unfortunately I think that FPSC is too closed to allow peripherals like that.
FPS movement tends to go by mouse movement rather than position, like in a FPS the system does'nt move the mouse, instead it uses the mouse movements to adjust the camera angle, so even a system that hijacked the mouse position would'nt work - it would really need to be specially coded.
However, the brave could invest in DBPro to develop their FPS (using FPSC as a springboard engine and level editor) and make use of a PC lightgun, there was even one sold here at one point, so I imagine that they're reasonably easy to code for in DBPro.
Van-B
Put those fiery biscuits away!