Here is the main circuit on a breadboard

The shift register on the left, driven by Arduino Mini Pro. This part has been tested and proven yesterday. The USB-UART converter is still attached to Mini Pro in case I would have to reflash it.

Mini Pro UART is connected to Micro Pro. Yesterday the only function of Micro was to pass the messages through USB (so no different than USB-UART) back and forth between PC host and Mini.

Today I have added the first part of joystick emulation using Arduino Joystick Library. Some of the button press/release messages from Mini are intercepted and converted into joystick button events.