I don’t see a real way of doing this without writing your own firmware, because playing is just one type of reading and the video is stored in files.
You could block playback of commercial region protected DVD Video by setting the drive region to something other than the local region, e.g. region 3 if you’re in region 2. You could even change the region the 4-5 times necessary to prevent further region changes by the user.
The user would still be able to use such a drive for playing region-free video titles, including all copied DVDs.