It looks great, Timoe!
Bones:
When I did my CP like this one of the things I had in mind was that I would set up all the individual game .ini files using it. Therefore I put in all the admin buttons I normally use: TAB, ENTER, ESC, PAUSE, F2 (advance), F3 (reset). As Timoe pointed out it's much nicer to connect these to an Ipac (or Keywiz) as mapping them to the GP-49 doesn't work if you unplug the GP-Wiz 49 and go to a keyboard encoder and then plug the GP-Wiz 49 back in.
Timoe: If I had the Ipac on my panel I would hook all the buttons and P2 joy to it and let the GP-Wiz 49 only be hooked up to the 49-way itself. This would provide the least amount of hassle mapping things in MAME.