Under the File Menu there is a choice <Load Playlist/Cuesheet> that will load an .M3U or two other filetypes into Mp3tag. I'm wondering why this doesn't appear in the context menu. I added it manually and it works fine. Thus:
"C:\Program Files (x86)\Mp3tag\Mp3tag.exe" "%1"
P.S. I just looked at an old post from 2004. Apparently there were context menu entries back then, but somewhere along the line they seem to have been dropped.