As you say, playlists in RMM don't have such properties.

What about genre? What soup have you gotten where you want to arbitrarily set genre across the board? I can believe that you might, but I'd think that it'd be better to ask. For example, I don't know that I'd say that "War", "The Joshua Tree" and "Pop" are all the same genre.

What about REM/Rem/R.E.M.? How are you getting to a soup that contains that set of artist tags that doesn't contain other ones you wouldn't want to also change?

I've got no problems with the idea of doing the propagation, but it shouldn't do it without asking.