Check out the video:
This thing is awesome. You could mount an inductive sensor, near the bit. Put this thing on, tell some software to map out the top of your workpiece, as the metal pin pushes into the material, it moves closer to the inductive sensor. The sensor trips, resulting in a height measurement. Then you’d remove this thing, and adjust the overall height to a particular place (XY), w.r.t. the bit, and the firmware would have to follow the z map when milling. It all seems doable, and more reliable, more automatic than a touch plate you move around. The nice thing about a touch plate is that the height of the bit is also measured, but if you’re using a carving bit, for example, I’m not sure I would want to probe that fast. I’m still not sure exactly how some kind of manual mapping is done in the firmware. I would need more understanding of that first, at least. I know some CAM programs (like estlcam) can probe a surface, and then adjust the gcode to apply that map, but I think Marlin can too, and then you can use any CAM software.
Anyway, it’s at least useful as inspiration, if not directly applicable.