Good question and you have answered one of mine as I plan to use a solar trickle charger as we have heaps of sun out here and was wondering if the dc sockets are live when ignition off on an X5....
I am sure the circuit can be cut and switch put in place to turn the dc 12v on and off but would not know where to start alas, you will get an answer from one of the gurus on the site I am sure, it's a great site.