To my fellow Linux Users -- tagging MP4 files?

[TonyPh12345]

Ahh.  I found a solution:   Atomic Parsley.

./AtomicParsley /path/to/media/file.m4v --artwork /path/to/JPEG-File.jpg --overWrite

…does exactly that, and more.

Now my script also reads the XML metadata from the WDTV content info and adds that to the M4V files as well.