Isn´t the surface flat? No problem with a DATRON CNC-machine M8Cube and an integrated probing with Z-Correction.
DATRON customer, Rapid DTM, had a run of 8 aluminum valve covers come in that required custom engraving. But, the surfaces of the valve covers were not flat -- there was about 0.02" (20 thousandths of an inch) of variation just in the area where the customer's logo needed to be engraved. No problem with a DATRON CNC-machine M8Cube and integrated probing with Z-Correction.