"Wonder if he could somehow get his one-button clicker to translate morse code into ascii."
Use the Arduino Leonardo to send native keyboard key strokes into the USB port of the Mac - it's robust and works every time - plug and play (once it's been programmed).
The Leonardo could be programmed to listen out for a specific pattern of switching and then send the entire password down the USB cable, or alternatively it could have some simple or complex feedback (lights / tones / onscreen keyboard display on a second mini screen) to allow individual characters/keystrokes to be sent down the USB cable from the Arduino.
I've been making stuff like this in the UK for the charity Scope, and their users - often people who have cerebral palsy. I could potentially make you something and post it over - if you are interested drop me a line.