Ok, I sometimes do my best thinking in that little space between sleeping and being awake and this is just a wild assed idea that I came up with after engraving rough bandsawn deck boards that have slight variations in height.
A laser range finder to locate zero on the fly and adjust the z axis accordingly. that is, as the laser dectects a change in the height of the workpiece the z adjusts up or down to maintain depth of cut relative to the surface of the material.
While this would not work on a sphere, on gentle curves, slopes and un even deck boards, it could prove inavaluable.
I honestly have no idea if the arduino or gcode could even support such an abomination. But just for kicks I present it to the community as a topic for discussion.