Visual Pinball and Future Pinball have a few fixed keyboard functions like Esc to exit. and FP doesn't seem to want to accept joystick encoder buttons for programmable functions.
Joy2key or xpadder might work, but I've never used them.

Keyboard-type encoder is the way to go IMHO, since you don't have to deal with reassigned joystick IDs (I know--drventure's got the fix for it in his sig) or other software like joy2key or xpadder.
Scott