Every time a song starts two random people on opposite sides of the crowd start blowing whistles. Not all at once. Just a couple random whistles at a time.
Just keep doing it through the performance; skipping around.
Wear a GoPro or have your whistle buddy film it.
If ANYONE touches you, that's criminal battery.
Get it on video.
Let’s combine our ideas. We write alternative lyrics, especially to “The Peoples’ Song” from “Les Miz.” We get these new Deplorables lyrics printed up and distributed to an organized group of counterprotesters. We assemble on the fringes of the crowd at this musical demonstration, with bells, whistles, drums, bugles, fifes, kazoos, and other easy to play instruments. When the Broadway Performers start to play/sing one of their numbers as a protest song, we break out in our version, loud enough to disrupt their performance, if not drown them out altogether. (We will need an amplified sound system; they will certainly have one.)
Of course, this would take some time and resources to organize. How long do we have?