I'm trying to put together a conreoller config file to use with my GP-Wiz that has 4 joystick inputs plus 28 button inputs.
I can't figure out from anything on the web how to map buttons above button 6 in the .cfg file. 
Any help is appreciated!
Have you looked at
easyemu? The first 1/3 of the page or so is all you need.
The easiest way to find out what these buttons are called is to (in a non-important game) press tab in mame and remap something to that button. Then exit mame and look at mame/cfg/
gamename.cfg and see what mame called it.