CNC Touch Probe
I recently saw the author of Gcode Rripper has added touch probe height re-mapping so you can carve on uneven surfaces. This will be great for carving on curved surfaces. Instead of building a touch probe, I bought a Mitutoyo Touch Signal Inspection Probe model 192-001 on eBay for $7. I cut off the original wire harness and soldered a wire to the circuit board that connects to the internal touch probe switch. The wire then connects to the Mach3 probe input pin.
Works great and a simple hack for only $7
Here is the setup I used to test cut a circle on a slanted piece of wood.