Did some further research and experimenting on this topic. The DVD-video format seems to be at least partially proprietary. But I did find some examples of DVDs that had other things besides the AUDIO_FS and VIDEO_FS directories, mostly Windows executable (.exe) files for unknown purposes.
So, I decided to try something out. Created a DVD with the tovid command line suite. At the last step, after creating the menu and dvdauthor XML file, I used
(without the -burn option) to build a copy of the DVD file structure on my hard disc. Manually typed in a short text file and copied it to the top level directory of the file structure. After verifying with
tovid dvd <xml-file>
that the amount of data did not exceed capacity of the disk, ran
du -csk <location of top-level directory>
The resulting disc, with the text file, loads with no problems on two different DVD players. So the answer seems to be "yes", it is possible to include some metadata text on a DVD video disc.
tovid dvd -burn <location of top-level directory>
Am marking this thread Solved, but will continue to experiment and report on any limitations of this method.