In the early days of P+ Howard wanted to book Richard Dawson. A letter was sent to a VP at ABC asking permission to book Richard. ABC nixed the idea.
Betty, Cullen and Dick Martin were in a special category of celebs who could fill in on short notice if a previously-booked celeb became "unavailable".
We had once booked CNR for P+ and were all set to go. Allen came to the studio on tape day, learned of Charles's booking and threw a hissy fit, calling Charles a "creep". Charles was summarily unbooked and another celeb called in at the last minute, either BC or Dick Martin.
Close the closet door behind you, Allen.
Barker, Dawson, Rayburn, Convy, CNR, Brett and the other emcees were under contract so the chances of their appearing on non-GT shows were slim, but there were exceptions. Orson was probably under contract for TTTT but I'm guessing that ended when the show ended.