At the closing performance of the original Broadway production of Sunday in the Park With George, Mandy Patinkin brought everyone out on stage at the end.
Sondheim closing performances can be kind of crazy. I've been at a bunch of them. I was not at the last performance of the Roundabout Sunday, but it was reported here that "Move On" got a standing ovation and the audience remained standing through the end of the show. Which would have driven me crazy (and which I think is kind of inconsiderate of those who don't want to stand or may not be able to stand).
I have been at a few closing performances where there was a speech of some sort at the end, but more often than not, it's just a particularly enthusiastic response at the end (and, in a musical, throughout).