Actually, it's not the pressure from stamping. It's the metal. Hard to scan through it. With soft metals, like gold, any nook you could put a chip in could get mashed shut or gouged off with routine handling.
Besides, there's no reason not to mint coins with serial numbers on 'em, or even barcode them with a slightly dissimilar 'contaminant' metal that would show up on a scanner, but be invisible to people. Such a 'barcode' could be done straight through the metal, so no amount of defacing would remove it.
Of course, once a number or barcode pattern has been *stamped* into metal, it affects the structure of the metal very deeply into the metal, and shows up in an XRAY like it was never filed off....
They could stamp that pattern onto the coin, THEN stamp the face onto it to conceal it, and you would be none the wiser, and only thorough mangling would remove the pattern from *inside* the coin. This would be the cheapest and simplest solution.
Besides, the government would probably make any "new" gold coins have a zinc core, like all the "silver" coins they put out now. That core could have any pattern they liked. The only way to remove it would be to file it down to nothing.
Of course, any defacement of money can (and does) render it technically NOT 'legal tender' (and it's a crime!), so once you've gone to all the trouble, you've only (legally) got a piece of paper or disk of metal.
Added: Solid state X-RAY scanners (as used in a dentist's office), combined with OCR would do the job nicely.
If someone IS routinely defacing money, an "account 0" could be assigned. Someone who's doing LOTS of this defacement will end up with a peak on "account 0", and could be caught at it. Their bills (and coins) won't scan. When someone has to manually read the money, they'll still get the number that can be noted (before the bill or coin is destroyed), and added to the account that originally received it from a bank. Now, once again, the matching and counting occurs over a period of time, and YOU have a big spike on "account 0", instead of a few "hits". Guess what? You're caught! First time, you get a warning about accepting defaced money, and it's filed away with your "records". If it keeps up, then next time, an arrest warrant could be filed.
Of course, this places a certain LIABILITY on people who withdraw money for other people, who then go on to deface the money. It all shows up on THEIR account. In addition, over half of all convictions occur because people don't know when to keep their mouths shut. They "confess" their sins to the police, who will not "forgive" them, and these words (as the Miranda rights do mention) will be used against them in court. So, a certain number of "warnings" will end up with prosecutions, (or threatened prosecutions for information about who was paid). Handy leverage.
