That's a good point. I have been looking around with no success, but are there any GPS/DVD head units that can display the NAV stuff to an external display while playing a DVD on the primary display?
The idea is for the upgraded HU to display the NAV on the OEM nav display so as to avoid having the cop-bait.
If not that, what about a HU that plays the video to secondary display while showing the NAV on the primary (reverse of what's described above)? This would still attract cops if used while driving, but I'm sure it would never be used that way.