Pete,
What my answer regarding a request to fill in a serial number on a head tube would be depends on who was asking and what the frame was. Anything that comes in here probably is something that should not have an original detail like that altered. A Pog should most deffinitely NOT have the number filled in. If someone insisted I'd think they were trying to hide something (like maybe the fact that they don't have rightful possesion of said frame); and depending on who it was I would insist on leaving the number on the lug or them finding someone else to paint the bike. If someone came in, proved they were the original owner, proved they were stupid and didn't care, AND gave me a good explaination why they really wanted it filled in, I would probably go ahead and do it under protest.
Brian Baylis
La Mesa, CA