I'm not sure I understand the goal here. Are you trying to learn more about how the original keyboard works, or are you trying to get your own code to run on the microcontroller in the keyboard? If the former, the easiest path is likely watching the interface while interacting with the keyboard. It won't take too long to figure out how it communicates with the host. The early Macs were minimalist to the core, it has to be pretty simple.