I think I'd be in favour of something that went in K: to say "what follows are to be interpreted as percussion symbols". It seems to me that what the key signature does is to define how the following notes are to be interpreted (this one is sharp, that one is not and so forth) and it's merely a generalisation of this to say "this one is a floor-tom, that one is a cymbal".
So my suggestion is K:Percussion or if you prefer K:clef=Perc or K:Clef=P and then everything is to be interpreted as percussion until further notice. Players should make banging noises, formatters should use x for noteheads, transposers should have a great time (*some* percussion is tuned!) and so on. Laurie To subscribe/unsubscribe, point your browser to: http://www.tullochgorm.com/lists.html