I did a bit of searching for this as I have the ability to play DVD and iPhone videos while the car is in motion but couldn't find anything. I'm not sure there is an easy solution. I think you'd have tap off the video out but you might get issues with the main iDrive screen if you do that. There is also the slight issue of not being able to see nav on iDrive and video in rear simultaneously. I can see my back passenger getting annoyed if his cartoons keep changing to maps
I decided to go for an iPad solution instead but let me know how you get on