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.

That’s fine when you’ve already locked the price into something else that’s not-btc.

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?

The issue is finding someone willing to loan you $20B for your bitcoin without doing the research of how that would tank the price of bitcoin.

It would be a great deal for Satoshi, but no other party would accept.

It's not just about the provenance. I don't think the bitcoin market can stomach a 20 billion sell-off, no matter who does it.