What about something like Microdots which can be applied to the frame and other parts? They seem to be common in places to deter theft of cars and other items
I've had two bikes stolen, both correctly registered in every relevant registry and one with microdots. Not a word.
The economics are extremely simple: There is a significant market for "don't ask, don't tell [if they're stolen]" bicycles. The same probably exists for cars which is why it took immobilizers (not frame/engine serial numbers) to curb theft.
A further complication makes a bicycle immobilizer impractical: It is trivial and not even very conspicuous to carry a bicycle quite far, or stick it in a car. That allows you to remove the bicycle to a private location where you can remove or destroy the immobilizer - if you could do the same with a car, I'm sure immobilizers would be less effective.
> If the police or a good Samaritan find your lost or stolen assets marked with our microtag technology and contacts MyDataTags, we will inform you and give you their contact information so you can recover your personal property.
So it's a serial number that nobody will notice (1mm black dot) or be able to read (I would assume it's gunk to be scratched off, not taken to with a magnifying glass).
Maybe I'm missing their key point, but on a bicycle the reasonable responsible buyer will just run the serial through whatever stolen registry is most popular in their area, not scour the bike for tiny secret illuminati codes.
http://www.mydatatags.com/