The site which is linked from erikson.no (http://www.cus.org.uk/~cleggy/) is a really good resource for the Sony UniLink protocol. They they emulated the Sony device to allow AUX in's on a Sony headunit and this is GPL at sourceforge.
The theory behind controlling a Sony CD Changers from a computer is there. They know the commands it just needs more work.
PIC chip interface between EMPEG serial port and UniLink changer. Hardware sorted.
Now can the EMPEG kernel be hacked to send the button press commands through the serial port as ACSII ?
Thats how people are controlling the MP3 playing with the Sony steering wheel sticks isnt it? Yeah, thats reversing it, but is it possible?

Cheers

Dave
-- Anticipating empeg fun...