Yes, canbus is a good idea, it seems. While considering the various options to install some tuner in my car, I saw that Becker's head units are in fact canbus, apparently, and when replaced to Audi's head units they still display all the computer info and mobile phone info on the screen.
All this... on paper. Never seen it working yet
If only one could split a canbus connection and use it for some external player such as the Karma/iPod etc, good chances are one could even use the factory steering wheel controls on those... Really nice...

A Karma that could be integrated that way would be a dream. I'd buy one tomorrow (provided you can replace the hdd).