I finally found the Ad for the spring loaded engraving tool I was referring to. Claims it can engrave curved and uneven surfaces. But I guess I was wrong. By the looks of it there really isn't any depth control as such. Just that it can generally follow a surface.
http://www.2linc.com/spring_loaded_engraving_tool.htm
This design could easily be improved for PCB milling by modifying it to where the engraving bit has a fixed stickout from a ball shaped spring loaded nose of the tool. The one shown in the link might even work if the engraving bit can be set back in the collett to a few thou stick out. The nose could then press down on the surface of the PCB material. Seems like it should work except for the driving over the swarf problem. Maybe I should design one.
Thoughts
Sage