I'm not sure you understand the application.  (or maybe I'm confused!).
The mayflash adapters seem to be for using console controllers on a PC.  There would be no PC in this application, or console controllers.
My idea would be to use a jamma wired cabinet, with an xbox and VGA monitor(or LCD).  No computer / no controller hacking required.
I realize we are lacking the documentation to know how exactly the xbox buttons show up on the jamma harness (I could make a guess for UDLR, 3 buttons, and start).  Perhaps they use the semi-standard 4th button jamma pinout, and put the white and black buttons on "test", "service" etc.
But for most the emulator on xbox, you'd be ok with the limited buttons.