My "Brother from another Mother" uses rear window defogger repair paint(?) to repair broken or burned traces on PCBs. Once upon a time he repaired my then new, biggest, baddest video card available (ATI Radeon 9700 Pro) after one of the traces to the PCI slot got fried. I couldn't send it back as it wasn't the manufacturer's fault and I had heavily modified the card. I also couldn't get another $350 past the wife.
Wouldn't surprise me if the inventor of the circuit scribe might have used similar techniques in the past.