That's a good interfacing issue. Particularly for people with tablets! And honestly couldn't be that difficult to implement in a good way.
The main problem (that I've observed) with notation on touch screens is that it's a pretty precise act, and your finger is not so precise. So usage on a tablet would likely be optimized for a stylus or something. But honestly, determining what someone is drawing, since notation consists of mostly simple shapes wouldn't be (theoretically) that difficult particularly with today's tablet technology.