You might want to ask Mike Craven, who posted the terrific
MINI Video Input article in
www.motoringfile.com -- I don't recall him talking about such a 12v feed. I think you're right that the Nav system uses a 12v line that's always available, and detects the key in the ignition from a message on the I-BUS or the K-BUS.
Mike uses a 12v line that's on while the car is turned on and the 12v line from the reverse lights, and added two inexpensive Radio Shack relays to enable a rearview camera automatically when in reverse, and a composite video input when he flips a switch.