Who owns the Grand Rights? If the production company does, then the music does not belong entirely to Wildhorn and his collaborators. The production company would share in the subsidiary income if any. The same company would be responsible for paying the actors what might be due them for in this case, not making the album.
The Grand Rights are usually controlled by the production company until the last first class production closes. I cannot imagine the producers ceding the music rights in toto back to Wildhorn.
Legally it sounds like a triangle: Actors vs production company and then the company vs Wildhorn to recover those labor costs that they had to pay to the Equity talent.
If Wildhorn really wanted to save money, he'd use synthesizers and pay three musicians to do the work of fifteen.
But he wouldn't dare try that with AFM Local 802. Or would he?
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher