Actually, using the "Halleluijah Chorus" sounds like evangelism to me. It may be done with the purpose of evangelizing for good culture, but the words carry the power of the Gospel.
In 2003, the community theatre group in Washington, PA performed Jesus Christ, Superstar as their annual musical. The director, a Presbyterian, was not content with the way that the musical ended with the crucifixion, so the entire cast sang the Hallelujah Chorus as the curtain call, with the actor who played Jesus taking his bow in a bright, white robe to signify the resurrection. From the comments afterwords, and what little I could see of audience facial expressions from my position on stage on some very high scaffolding, the piece did have the usual effect on people.
You are absolutely correct that the words of that work carry the power of the Gospel, and are therefore a form of evangelism wherever they are sung. I am very optimistic about the positive results of such flash mob stunts. I am equally pessimistic about many Lutheran congregations getting off of their collective seats to actually engage in such actions. I wish more of them would, but I am not hopeful that it will happen often, or in more than a small handful of locations.
