Look at engraving systems such as Gravograph, a spindle system with a depth guiding nose surrounding it is used to achieve consistent depth on flat, curved and irregular surfaces.
I've built a sprung slide for my benchtop CNC mill which is driven with a pulley from the main spindle to utilise a micrometer depth adjustable engraving spindle which allows me to engrave 2, 3 and 4 axis jobs.

- Nick