I've added these, with mostly the terms suggested by @ListMyCDs.com.
The Lyricist rel now includes:
Lyricist is to be used for the author(s) of the words of both popular and art songs, including songs from musicals. For the author of the dialogue from a musical, use the librettist relationship instead.
The Librettist rel now includes:
Librettist is to be used for the author(s) of the words of an opera or other musical stage work, or an oratorio, including dialogue from musicals. For the author of the lyrics of the songs from a musical, see the lyricist relationship instead.
and, only at the work level:
For musicals and other cases where the author of the libretto didn't also write the lyrics, link the librettist only to the main work (and any act works), but not to each song.
Both release level rels now explicitly specify that
The work level relationship is preferred and should be used if possible.
This does still leave a grey area (for things like mentioned on the previous post) but it does specify the expected usage for musicals and the like and also that lyricist rather than librettist should be use for art songs. Further improvements and discussion definitely welcome