Hello Budgie2,
this is possible by using the “Edit relationships” tab. You can enter the relationship type (probably “vocals”), the Artist name, and the type of vocal, i.e. Tenor, soprano, etc. I always put the role filled in the “credited as” box next to the type of vocal.
This works and nobody is complaining. It is somewhat cryptic and the information is usually available somewhere.
Have a look at the relationships of one of my recent submissions.
https://musicbrainz.org/release/a391dc65-57e7-4e01-a9f0-99c1cb86b83d
Regards,
Hollander