2. I discovered that the TVSHOW tag shows up for m4a files (unprotected) in mp3tag but there is no TVSHOW tag on the mp3's. This seems to indicate that itunes stores this information differently depending on the file, which is a bit messed up.
That last item probably means that the tag is somewhat non-standard, but I figured that it should at least show up for the m4a's which it doesn't.
iTunes stores some of its metadata only in their database. Another obnoxious one that they make very difficult to use is their equivalent of MC's [Media Sub Type] (called Media Kind in iTunes), which is only stored in the iTunes database, and not exposed through the iTunes API.
That makes it difficult to sync Audiobooks back and forth between MC and iTunes because there's no way to tag them as Audiobooks in iTunes programmatically, even using a third-party sync tool like prod's MCiS. You also have to manually tag the Media Kind on Movies and Shows you import from MC to iTunes.
I suspect Apple's argument (much like JRiver's, in fact) would be that there aren't good tagging standards for these kinds of things. I'd still rather it expose some way to set it, since they force you to sync audio to iDevices through iTunes. But, alas, it is quite difficult. I think you could do it by reading and/or modifying the iTunes Library XML file directly, though I've never tried to wire anything up.