Using the pedal as is would be pretty impractical to try and piggy back off the aimtrack setup for a button. You'd be better off just hacking the pedal directly to the PC assigned as some other key.
So long as the pedal uses a switch of some type, wire that to a single Key stroke and in Mame assign that key as the reload, or "hide" or whatever button (should work I think).
I've modded the guns before, and I've created some more complicated pedals that use potentiometers, but for that I hacked a separate joystick.... so ... it can be done, it just depends on how much you want to customize things.