Cool! It may be useful for @proof.ink to further timestamp the hash using https://opentimestamps.org, which is a free timestamping service that anchors to Bitcoin. It takes about a day for timestamps to complete (get encoded in a Bitcoin transaction). When this happens, @proof.ink could post the OTS stamp on the Steem blockchain. I expect timestamping via Bitcoin will be considered the most definitive form of timestamping going forward.

Perhaps you could also just timestamp the Steem Blockchain's block hash every day using OTS to provide additional auditability.