I'm not a gamer but I may be able to shed some light on this subject. I once built a custom keyboard using a super expensive all-metal keyboard I bought surplus for $50. (mostly because it was so big and looked so cool). It was originally made for an ATE. It
was a parallel output keyboard. I took it apart and found where the cable connected to the PCB to isolate the location of the parallel
pins. I had no documentation so I had to wing it. I connected wires to the output to drive leds and pressed keys and looked up the ASCII code for those keys. By using certain keys it was easy to determine the MSB and the LSB. Then I wired in a UART (parallel to serial converter chip) and used the parallel lines to address the UART and generate serial ASCII. Then I inserted a PROM between the parallel output and the UART and programmed the addresses to output STANDARD ASCII instead of the custom keyset that it originally had. I pressed a key , the address lines addressed the memory location where I burned the ASCII I wanted to assign to that key and the output of the PROM was connected to the parallel input of the UART resulting in a standard ascii serial keyboard with
dip-switch progammable baud rate. The electronics may be simpler than the physical replication of the XCM. If you can replicate the functionality where are you going to connect it ? (what device are you going to hack ?)