Interesting! I once wrote an excel macro that used checksums to validate bank accounts, and I cannot remember anything about them except that it was just maths. So I'm guessing the websites just use this format as described in your link.

In your link they are speculating about invalid numbers. If they still have stacks of them in the shops to hand out to people who would want to use them right away, I doubt they would reject an unregistered card. Just record it the same way as any card but only allow points or whatever to be spent when it's been registered.

I've used the cuntdown site a lot and can only remember one time the barcode was rejected.