My build (now with secondary shoulder buttons!)

[Flavor]

I've been working on a new version of the mk_arcade_joystick_rpi code.  I hadn't realized that it basically wouldn't properly accept/use the GPIO lines above 31 that the CM3 has.  I had to re-work a bunch of the low-level stuff to accept input numbers above GPIO31.  I will be testing it more, but my code is now using GPIO40, 41, 42, 43 as extra buttons.

Flavor, I'm looking to add L2/R2 buttons as well as a PSP analog joystick to my build, via the GPIO expansion points on the board. Eagerly awaiting your results to see if its possible!

Yes, the support should now be in https://github.com/TheFlav/mk_arcade_joystick_rpi and in the latest SD image at http://freeplaytech.com/build for the extra buttons.  You can just add them with a config change /etc/modprobe.d/mk_arcade_joystick.conf

See the line that's commented out in https://github.com/TheFlav/mk_arcade_joy...install.sh for an example.

As for PSP analog, we're still working on that.  It will need some extra hardware to do the analog->digital conversion that the Pi can access.  We're working on all that.  The pieces have been validated, but it's not really in a state that's usable, yet.