You interpret your hash as a private key. Or as a seed for the private key. Then out of the private key you compute a public key, which when hashed with SHA256 and RIPEMD-160 gives you an address.
Then, before disclosing the document, you transfer some amount to that address and back to your private address. After that, reveal the document: now everyone can repeat the procedure and verify that you have created that document (because you can always prove that you own that "coin" which travelled through the synthetic address).
Wow! I didn't realize that you could generate a public key given a private key. To me, keypair generation was just a "black box." I should have known (probably studied it in class.) Thanks.
Then, before disclosing the document, you transfer some amount to that address and back to your private address. After that, reveal the document: now everyone can repeat the procedure and verify that you have created that document (because you can always prove that you own that "coin" which travelled through the synthetic address).
You can play with keys and addresses here: http://brainwallet.org