Hiya, I did a search on the interweb which resulted in the keycodes for s etc.

Not sure what the keycodes are for actual hardware keys on the 770 though; I did keep a list of the websites I searched on the 770 - but I'm at work now with no 770!

It's all findable; it took me about 2 hours on a Thursday evening in front of the TV, eventually resulting in the right solution for my external gamepad, where I'd set the driver to map the "start" button on the gamepad to "s".

Further to what you are trying, it's probably correct (or at least on the correct track) - may I assume that you're putting the hex values in the xmamerc file instead of the literal text? If not, I'd suggest you try the hex values.