I'm repeating what I said above, but just send yourself an gmail with the hash in the Subject. Gmail will kindly timestamp it and provide a DKIM signature. Publish the mail headers gmail includes in the signature (which includes the timestamp and subject, but not the contents), the signature itself, and a link to hashed the document and you're done.
This is only true if Google never release old private keys for DKIM signatures, which various people have been campaigning for them to do in order to provide long-term deniability around DKIM-signed mails.
> This is only true if Google never release old private keys for DKIM signatures, which various people have been campaigning for them to do in order to provide long-term deniability around DKIM-signed mails.
