Hmm, It doesn't seem like it for me, I added just one tag on the "official list", that is "artist", a sidecar-file was made immediately, and when i tried to play the file on a MC-install on another computer without the sidecar-file, the artist was not populated. IT also should write non-ifficial tags to the MKV-fiels imho, MKV supports custom tag-fields.