Atari ST USB keyboard, I can reuse that and read in the data from the keyboard. It is a custom protocol, but there is quite a lot of documentation and code around on the internet, The data is in the form of key pressed / key released information, so that is used to populate a matrix of keys currently pressed, and that is then used to complete the USB keyboard report data.
9 way D USB Joystick adapter, as the pin spacing for the RJ10 socket fits nicely. The 4 pins are ground, clock, data and power, these are wired to the fused USB power and the data pins to two I/O pins on the microcontroller. These signals appear to be open collector, as both the keyboard and the Mac can drive the data line, although the keyboard always drives the clock line. I've added pullups for these on the bottom of the board.