Yeah, that what I thought but it doesn't feel completely right to me: it looks a bit messy if you have multiple performers but more importantly I would like to retain the information that the music is improvised (which is lost if I just put the performers as the artist.)
In the release I'm working on, they actually list "Improvisation" in place of the composer name in the booklet, e.g. in the following the two first tracks have an actual composer (Jordi Savall) but the last is an improvisation:
- Cantus Caravaggio I (Jordi Savall)
- 1 Lachrimae Tristes (Jordi Savall)
- Deploratio 1 (Improvisation)
Wouldn't it be better to do the same and create a new Special Purpose Artist '[improvisation]' or '[improvised]'? The performers can still be listed in the relationships.