I would love the addition of lead sheets, but not before discussing it's application to a pulp. For starters, lead sheets are diverse in that they are often offered in popular transpositions for common transposing instruments. To do this right, I feel that we should include C (treble clef), Bb, Eb, and bass clef, and if we're being generous, F. Your idea above works well for duets and other versions, but do you have any suggestions for how to organize these transpositions? I would imagine the easiest thing to do would be to list them all as separate entries according to their transposition, but that might be a bit barbaric.
Also, is there a way to have different transpositions for the same chart not bump the total sheet counter? Technically these will all be the same sheet, just for different instrumentalists to use. More later.