Zwilnik beat me to the point about the volume switches not being gaming buttons (they're also intentionally difficult to press, and for good reason). Ultimately, the problems you've outlined with FPS games on these devices won't be solved by hacking the volume switch or any other built-in physical button.

ThemsAllTook's got the right idea - you need a peripheral, but I'm not convinced a game pad would do it - they're basically as crap for FPSs as a touch screen (it's just that people were forced to get used to them). An FPS peripheral needs to be more like a Belkin Nostromo mashed with trackball. Or some crazy contraption that creates a DS-like iPhone sandwich where you use one for controls and the other as a display.