Calliopes for the most part utterly lacked different stops or registers such as you might find on a bellows-driven pipe organ. Mechanically operated ones worked in much the same fashion as a music box or player piano, with a "roll" of some kind specifying notes in a sequence to the player mechanism. ABC notation in Books, which could be placed in a chest/GUI slot, could be a nice way to avoid relying on other mods for note sequencing. Another, more elaborate method could be constructed from vanilla redstone-and-piston logic, causing Redstone Comparators to send variable signals to the Calliope. That would get you about an octave and a third right off the bat; a shift bit would get you the traditional 32-note range of a calliope. Rests and note durations... I haven't actually thought it through in-depth, but one of those piston-based memory cell contraptions might get the job done. Of course, this would be a much bigger engineering challenge to get the darn thing to play something. A computer mod or maybe MFR could simplify such an undertaking.
Using ABC notation in Books would have the advantage of allowing players to copy and distribute their works while providing at least partial access to the huge body of existing music in ABC notation. The Calliope could read its subset of ABC notation from a Book into an array, etc. It's been a long time since I wrote any code, and I have never looked at Java, so I reckon you'd know better than I would how it would look from that perspective.
If a Book is an inappropriate or too-general data structure from the Minecraft side of things, then perhaps something custom in its place?
