Would it be possible to add an exclusion character set to the multiple artist splitter (maybe curly braces, {} )?
So when tagging older material where session musician details cannot always be 100% reliable you can tag an artist | role/ instrument as:
Ernie Newton {poss.} | bass
The full info goes into the tag, but MB only parses the 'Ernie Newton' bit into the Artists list.
That way you don't get 'Ernie Newton' and 'Ernie Newton {poss.}' as two separate artists.