I imagine in said document you could simply reveal Y & Z but in theory, an attacker could then cycle thru known addresses until one hash matches,
You can't do that theoretically, cause the document would be unverified then, it could pretty much be any signature for any address, or not be a valid signature at all, if there is no address with which to verify it.
You could provide the signed message and then encrypt that using a PGP signature, this keeps the information covert until such a time that it become needed, at which point you can decrypt it.