In the ID3 Edit pane, there is a Musicians Credit field (TMCL). Currently, it seems to be used as a single value text field. It should work the same way as Involved People (TIPL), storing role/artist tuples.
I use this field, albeit rarely, to track notable musicians contributing instruments or vocals. (In the long run, it would be nice if Jaikoz could pull down the various AR’s to populate this field.)