Knowing that it was a widespread practice definitely makes it seem more likely.
I was just thumbing thru Jeff Bond's book. Do you have this? Page 65 has some of what I was thinking about. That's in the middle of the Alexander Courage interview.
He talks about how 4 or 5 of them were doing music for almost all the shows & movies, and how after Shaft came out they were afraid that all of the work would dry up. There's a bit about him chipping in a march for a score that someone else (Goldsmith?) was doing. Some more in this vein on the next page, too.
You have to read between the lines a little bit, maybe.