Grendal_Fly, using the expensive Nav-TV video input "tuner" hack and playing the video on the Navigation screen is easily possible... though the controls won't be Touchscreen. Actually, you don't even need the Nav-TV hack if you're willing to use test mode. Just a cable with appropriate connector (or some soldering) to a connector on the rear of the Touchscreen. No need to prove functionality.
If you do have the DVD, I'm curious where the changer/player is installed, though. They aren't that easy to hide... and most won't fit in the glovebox. If you don't have rear screens, it's a little odd to install it somewhere in the rear...