TigerHeli,
What I've read so far looks good...but tell me if I've missed this...
MAME can be set up to use shifted input on it's own, i.e., "left shift and 5 not right shift," or something like that. Seems that would be of use to folks using a keyboard hack. (like me) Of course, this could involve a ridiculous amount of wiring...and anyone using a keyboard hack already knows that you've got plenty of wires to begin with! I believe this would generally be useful only for control keys, such as tab, esc, etc.
Like I say, I haven't read far enough to know if you've included this or not, so feel free to ignore me or give me 20 lashes with a wet PS/2 cable.
John