Earlier quoted context omitted.
Can he just pay off some kind of smart contract with it that ignores the provenance of the Bitcoin? Like, get a loan for $20b in a stablecoin with the payback being $21b in btc in a year?
If any of the BTC founder's blocks suddenly became active it would crash the price because most people would suspect someone found an exploit in the contract/hash or the keys were stolen.
While you could sign a message with your key, you would still need something that accepts the proof but isn’t public, which might be possible with a properly structured/constructed smart contract, maybe?