CoreXY is interesting, and seems especially well suited to plotters, where you don't need to worry generally about the speed of the tool. I've seen CoreXY machines doing circles at two or three linear meters per second (!) with print heads on them, I can only imagine how fast a light pen plotting head could go. Is there a reason why people aren't using thin cables or wires instead of belts at this point? If you used wires you could have a simpler crossover, I think (wouldn't need the extra pulleys); this could also reduce belt inertia, if that's of any importance.

Not sure if this is a dumb question, but what is CoreXY?