That is a HUGE group to coordinate! In Russia no less - I never picture it as being free enough for that.We DID do the Hallelujah chorus a capella with that group in Boston. The director noticed that - us, being used to singing it with accompaniment - were oh so visibly rocking and lightly tapping as a group and told us to stop it. I can still hear the trumpets in my head, toward the end - and our violinist during practice who would begin the chorus but end with "Yes We Have No Bananas" because they marginally sound the same.
But arguably my favorite flash mob is still the "Puttin' On the Ritz" one that happened in front of Moscow State University, if only for the fact that I stood in that very spot my last day in Moscow with my newly adopted son. (Obviously, years earlier).