To get the x-keys working is pretty simple (using the hid input in communications->stream control). 

For the programmable keyboard a MCI128 from preh, i choose a column & row approach  (buttons are programmed as keypress a+1,a+2 …till in our case  p+8). The incoming keypresses are decoded with a javascript actor. We had 128 possible keys, but you could extend this with another keyboard (starting with the prefix q till z) or extending the prefix with another modifier.

Attached is an izzy file and a key table we used for prehs config software "winprogrammer"

edit: // zip upload hasn't worked out, now a link  preh_decoder//