I have to smile when I read these posts. Many times here and on the US forums someone asks the question about unlocking the DVD playback. Many times the response is not an answer but shock and outrage that someone would even raise the subject matter.
The answer to the question you asked is yes. But only if you feel like removing trim, air vents, the ML system, snipping a wire, joining a couple of wires. Probably invalidating the warranty specifically on the ML (although they would have to prove the modification caused a fault, they would still be difficult with you though).
Now to join the righteous crowd, if you had an accident and this modification was discovered, might it invalidate your insurance? I wouldn't presume to know you and how susceptible you may possibly be to watching a movie whilst in transit, but I know myself that I may take my eyes off the road for just a few seconds to catch a good bit :)