I've read here that it is possible to modify the Phatbox by either adding on a serial cable port or by just matching pins on changer cable to a serial cable. If you modify a kenwood cable to serial setup, then get USB to serial adapter to control the logic. Would it then be possible to plug the modified serial PB into a USB head unit with for total control and functionality ?
The only user-visible application I know of for the PB serial port was one that PhatNoise released for the original Palm Pilot, that let users control the red (beta) PhatBox.
OTOH, the whole point behind the PhatBox SSA was to support head units that didn't have enough controls to effectively browse large music collections - hence the whole "pretending to be a CD changer and speaking" thing. If you have a head unit that supports USB storage media, why not just copy the music from the PhatBox to a USB HDD like a Western Digital "My Passport" and let the head unit do all the work?