One option I was thinking of, would be to outsource a sticker design that covered the whole board. Which layout had the numbers and connections exempt. So when the PCB is painted/paint marker/covered it would leave the number behind.
Side note: Vinyl used in the signage industry works VERY well (image below) for PCB making. It's cheap, all of the toner gets transferred, no need to soak and rub off remaining paper. - Scotty
Malcom,Pulsar makes a film that would do what you want. But, you simply transfer the film and leave it. No spray painting required.http://www.pulsarprofx.com/
I am interested in the technique using vinyl. Do you have a link for a tutorial or detaisl of the process?