My model S sucks 25-60 Watts sitting in my garage. You'd probably need a solar panel covering most of your roof to neutralize the vampire since you'd only be fighting it off in the daylight.
The only real benefit that I could see to doing this would be to increase the life of the 12V battery by reducing its charge-discharge cycles. On the other hand, there is the risk of overcharging the battery. And if the voltage gets too high, it could (though unlikely) even damage the cars charging circuit.
Regarding the OBD-II port, my understanding is that the U.S. requires one; even for non-ICE vehicles. My MS has one. And it contains CAN bus signals. My guess is that the M3 will have one too. But beware, it is fused with a 5 Amp fuse.
