Hi all,

I'm a bit late to join this discussion... 
This is just to mention some open source projects of interest regarding the music notation topic. First the GUIDO project (http://guidolib.sourceforge.net/) that already includes a Pd external and next the INScore project (http://inscore.sourceforge.net/) - already mentioned by João - which may be viewed as an extension of the former.

The GUIDO project is cross-platform (Linux, MacOS, Windows). It is based on the Guido Music Notation format, a simple textual music description language. It takes account of conventional western music notation and probably supports most of  the features you're discussing but no microtonal yet (which shouldn't be difficult to implement) and more generally no contemporary music notation signs. 
The GUIDO PureData external (https://sourceforge.net/projects/guidolib/files/PureData/) provides all of the engine capabilities but is actually not very efficient due to an implementation over Tcl/Tk (quickly made without prior experience of externals design). 

The INScore project is also cross-platform (Linux, MacOS, Windows). Controlled using OSC, build over the Guido Engine, it extends the music score to arbitrary graphic objets and gives a temporal dimension to all of them. It provides graphic synchronization between the components, interaction features... More about the system capabilities : http://www.youtube.com/watch?v=p96FVMENNtI 

Regards,

Dominique Fober 
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