I think the Plectrum tracks deal with guitar stuff.
But, not as much as you'd like. I understand what you're getting at ... and it can be done "simply" by defining different sequences of chord for different parts of the song. So, when you have a Am chord you'd set a certain number of note on/off, for Em different ones, etc.
Of course, if you transpose ... you're screwed
And neither MMA or MIDI are a real guitar/piano/saxophone