I don't know why you think that. JRiver reads and writes video tags whenever there are standards. I'm not aware of any formats that can be done that we haven't done.
I'm with Brian on this one. I've never seen anything other than "n/a" in the tag dump area for any movie that I can recall. I just checked a bunch of MP4s; Nothing. I added tags using Windows Explorer Details, "touched" the file and imported; The Comment showed up as the description in MC, but nothing in the Tag Dump area. I tried both Tag windows, and even added the "Tag Dump" Group and "Tag Dump" item in the new tag window customisation. Nothing shows.
I even installed MetaX, updated the tags for an MP4 movie, and reimported the file into MC. Nothing.
So, some tags in files may be read into the MC Library (i.e. Comments into Description), but nothing shows up in the Tag Dump area, so a user can't see what is in the file, vs what in in the Library.
I'm sure there was a thread about this not so long back. I couldn't find it in a quick search though.