Making a cnc "form relieving lathe" might be most possible with a Galil (or other) motion control system, so long as your spindle motor is also a servo motor. You could write a special routine in Galil commands to accomplish this. The movement would then become a 3 axis interpolated movement.
This differs slightly from a rigid tapping or single point threading, in that both lathe slides and the spindle would move together in electronic cam mode.
But running a normal 3 phase spindle motor as the third axis would not give tight enough spindle control to give good results, IMO.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)