I would use a combination of one of the fir buttons + start. That's what I use as ui cancel in mame (for quiting emulation). I've been trying with joytokey and it seems to work. I assing for instance shift to one button, and F12 to the other. Then I create a shortcut to a program to exit windows called af1shutdown, and associate the combination of keys shift + F12. The problem is that I don't know if this will have side effects. I mean having joytokey running, and registering shift every time I hit the button during gameplay. Also, I have to test this: I also use the same combination of buttons in mame to exit emulation, so maybe it also tries to exit windows apart from quiting emulation, which would be a problem.