I would suggest looking into the Model-View-Controller
(MVC) pattern.
From what you describe, you want multiple views of
the same data, each
updated in real time if you edit one of them.
I doubt there are many existing tools that would be useful for this,
but like someone else mentioned you could probably re-use and existing
text editor widget for the text view. I don't know of any text
editors that let you reposition the cursor from another process (like
over a local socket or something), but it's possible some exist.
Again I am getting the the idea for playback issues but what about
ascii to graphic updates so that a c''4 written in a text editor will
display as a third space quarter note on treble staff- and a third
space quarter note in denemo will appear in the text editor as c''4?
thanks
Aaron