I had an idea for the mouse dragging that I don't know how hard is to implement. How about making the object behave like the number atom, that is, if the user click and drag, the notes will go up and down chromatically (or in
the scale choosen) and if the user shift+click+drag the notes would move in a microtonal way. It doesn't have to be that much precise (I would say, two float points in midi scale maybe? = cents), if the user really wants to
make precise notation it would input it as a float on the inlet. At least in western music, this much precision is impossible to execute by a human player playing an instrument, this precition would be only for
calculations anyway.
I didn't see Mathieu's object yet, but just a small opinion about
mouse-dragging (or mouse events): afaik, the graphical display of musical
elements in open music et al is just a display, just an expressive way of
looking at numbers. I'm not sure if these surfaces allow for mouse control
in that way (I suspect they don't). Adding mouse control might be not
really necessary (if you control the numbers), and can make things really
complicated to manage.