The Spring Awakening Revival from Deaf West famously performed after they had closed after raising the money to do so. There have been many example in the past, for example Side Show and Rags, the latter of which was represented by Judy Kuhn who 10 minutes later also appeared in One Day More from Les Miserables. I think she was the last performer to perform in two musical numbers from different musicals in the same evening.
It used to be more common for people to not show up to award shows. In today's media environment, it has become more usual for people to be there.