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c) A signed QR code balance sticker generated by Casascius, to place on the new coins later.
I believe the reason that the codes aren't etched into the metal is because common medical imaging equipment could read the code without removing the holo.
Regarding point c, can you elaborate? If you're suggesting a second public/private key holo that the purchaser could apply after the fact, I don't think that would ever be something that'd be offered.
Re. Durable marking, this might be difficult or impossible; but some sort of temperature stable , subtle marking of the metal would improve the value.
For C, I'm imagining a separate code, not for the keys, but indicating the balance as verified on a date. It might make sense to store these in a coin holder with a pocket, to document the coin's history offline.