You'd have to put microdots along the entire length of the actual copper conductor and every single wire would have to be registered somehow, at least the owner needs to keep track of the numbers. I really can't see that being used on a large scale. If only one end of the conductor is microdotted, thieves will soon know that, check for microdots using a blacklight and cut the offending short piece of copper away.
Forcing all scrap yards to only accept dotted copper would be an even bigger effort, besides, there will be un-dotted old copper wires for quite some time.