I think iTunes always present such albums as VA (corresponding to the 'iTunes compilation' checkbox in track setting)
No that's something very different.
I am talking about albums such as:
Janine Jansen - Bach Concertos
Eno · Hyde - High Life
Those are certainly not Various Artists albums.
The first could be considered to have Janine Jansen as the album artist. (some might prefer it to be J.S. Bach)
But both Janine Jansen and J.S. Bach are album artists here.
Ditto for the second example, where Brian Eno and Karl Hyde are both album artists.
The same for the examples in my start post.
Elis & Toots is not a VA album. Some songs contain them performing together, some individually.
So the album artist would be Elis & Toots, just as it says on the cover.
The album artists are Toots Tielemans, and Elis Regina.
MusicBrainz already makes these album artists available (e.g. by means of Picard), and players such as Kodi (previously XBMC) also uses 'album artists' for many years now.