Doh, OK, re-running the script after rebooting fixed it. I must not have rebooted since updating the kernel version. Now I can run jstest /dev/input/js0and see the buttons work as well as shutdown view soft power switch and see LED indicate status.
I'm not sure if this post will be useful to others, but one never knows. In any case, the process of writing it out probably helped me figure out what went wrong.
I have just pushed an update for the ControlBlock driver. You can now connect a button to P2, "A": Pressing that button will lead to a simulated press of Start + B, as well as a keyboard button press of "F1".