The rear-view camera kit depends on a video-control box, which also has to be purchased. That control box alone is not nearly as much as the TV kits, about 200 Euros, and can be used to patch in a composite video signal. Mike Craven explains how to do it in an article on motoringfile.com .
Video Input for MINI
Depending on whether you get the video source's power from the reverse light, the parking brake wire, or just a 12v supply, the video works while you're in reverse, a complete stop, or whenever the ignition is turned on, respectively.
He set his up with a couple of relays so the rear camera overrides any other video input when he's in reverse, and can choose whether the screen displays another video source or the Sat Nav system.
What I've been looking for, and I think you are too, is a way to patch into the analog RGB input on the display. Some kind of switching box that passes through the Sat Nav video but switches to the computer when its output is on, or just a simple switch so I can select it manually.
Paul