Hey, Triffid, I think you might be onto something!
If you were to print out a copy of the PCB layout, trace the "traces" (nope, no pun intended) with that CircuitWriter pen, then quickly make an impression on a blank, you might basically be done because the board would have the correct "mirror image" of the circuit. Instead of going over traces with a Sharpie, we'd just be touching them up with that CircuitWriter. Of course, as Pete asks, is it solderable? That's the next question.
Can any of the board etchers confirm or deny this?