Calling it Musical BS would surely mean that the people who filter out the BS threads would lose them as well. (Mind, I can't really see the point of filtering them out, when it's easier just not to open them if you are thtabway inclined - in any case as often than not they aren't marked BS by the people who start them, which I think is a pity.)

I think the best thing to do with new prefixes is for people starting threads to try them out and see if they fly. If they catch on, they can get incorporated into the infrastructure of the Mudcat.

Music.misc might well be worth trying out this way for the threads that just don't fit in the other existing categories. Though theoretically if it's not BS or any of the others the assumption should be that it is Music.misc anyway.