I know he could sign a message from an early Bitcoin address if his PGP key is useless, but I'm curious whether it is or not.
Yes, he would need to sign a message from a few (more than one, the better, to prove the signer isn't just an early miner)
of his early mining bitcoin addresses. That would be very possible.
In relation to his PGP Key, many have said he had no such key, but there is this record according to bitcoin.org github:
https://github.com/bitcoin-dot-org/bitcoin.org/commit/19e0c74df2162d4510db5df9e50d5ac53b38c498
An alternative would be to have him sign a message with an address in the coinbase transactions in any of the first couple of blocks.
Edit:
I want to see if the two keys match or are two different keys for Satoshi.