Our USA based 2008 S 600 plays DVDs as long as the car is in park. It will hold 6 DVDs. This is new for 2008.
Our Bangkok based 2004 S280 will play a DVD from the dashboard slot if the car is in P, N or in D but not going faster that 5kph. The under 5kph allowance is a bit odd.
The European W221 plays DVD also up to 5 km/h or so, I'm pretty sure the US car could be programmed the same by DAS. Another issue if it makes a real difference, as you say. Perhaps at a parking lot, if moving the car just a bit while others are watching DVD or TV.
Have the US laws or regulations changed recently, I don't understand why they ripped off the video capability from the early US cars?