You really don't own shit as the musician unless you self publish. So as Kylotan said you will have to talk to the publisher about licensing.

and assuming they've turned a blind eye to the artists, chances are they don't want their music to show, as it would reflect back on them, which, presumably, they're hoping to avoid remembering anyone they were in business with the artist.

Sounds like a doomed approach, sorry.

Musicians that are signed to smaller labels (or are not signed up with any) can often do custom music relatively cheaply. Even those who have signed with a bigger label but who have been dropped may be able to do some work, which gets them more money than whatever the big label provides.

It may not help with the "I want it for free" people, but I've worked with several over the years to get some great music for very little money.