If you had a motor that was powering a spindle other than directly, would it be possible to have the motor fixed while the spindle moves? Vertically only over a short (say 50mm) distance, no horizontal travel required.

More of an idle thought that anything else, but it's been bugging me all afternoon. With pulleys the tension of the belt would change while running; what kind of impact would that have?

Oooh, what if there was an idler that moved in such a way as to maintain the same tension on the belt as the spindle moved up and down? I've just given myself an idea there, assuming it's a viable option...