"Little Do They Know" from The Act is about how hard dancers work behind a star, making the star look good, yet the star gets all the applause and recognition. Not quite the same thing as you're looking for, but might convey a similar theme.
Not sure if this would work for your show, as it has lyrics specifically about being Christian Borle's understudy (maybe there are alt lyrics out there?) but "Understudy Blues" by Amanda Yesnowitz and Brian Cimmet: