This thing is working like a charm!
Typing this on an A500 keyboard, that's really a wonderful feeling. Thanks, guys!
Got a question. In earlier versions of the Arduino IDE, one could simply add stuff to HID.cpp, but in the later versions, it seems like this has been abstracted into separate places, such as Keyboard.h and Mouse.h. Any ideas where to add the Atari DB9 joystick information nowadays?