Hi Chris, thanks for the interest! The boards were self-made, but I imagine that with a little time the Eagle files could be cleaned up to get the PCBs properly manufactured. Alternatively, it would be possible to jury rig a commercial board such as a Teensy to connect to the Advantage's microcontroller socket. (This would give the additional advantage of hardware-based USB, but would require bringing the somewhat neglected LUFA-based USB core in the firmware up to date (lufa/lufa_main.c)).
Modifying the firmware to perform extra actions on specific keyboard combinations would also be fairly easy. For a really quick hack, you could just toss handlers into handle_state_normal() in keyboard.c, otherwise you might want to look into using programmed-macros for the feature.
Unfortunately, I'm not in Wellington any more, otherwise I'd offer to help you build one. In the last couple of months I've moved to Tokyo for a new job, which has somewhat reduced my available time for hacking on the keyboard project.