I am struggling myself with the same question. As far as I can tell, some USB keyboards will function properly, but I guess most modern keyboards will not (not sure this is correct though).
So… I guess I cant help at this time.
As far as I can tell, some (most?) older USB keyboards will work with PS/2 adapter, but newer probably not.
If there is anybody out there with some hardware knowlege, that can tell us more about the diferences between these two interfaces, maybe we could come up with some hardware modification (strictly USB keyboard modified for PS/2 keyboard.
Already did somme digging, but nothing useful came up for now.
Regards