My question is, how does improvising a melody or harmony (much less through loud speakers) help to lead the congregation in singing as one? If it doesn't, perhaps the problem isn't with performing music according to its musical style, but with the musical style itself.
I'd say on the Mass of Creation Gloria, go nuts since everyone knows it anyway. You can accompany it in f# Major and most Catholics will still sing it correctly; it's built into their DNA by now.
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