By the way, this is the way most digital notary services work. You send them a hash of a signed document and they log it permanently. Then they create a hash chain like bitcoin does. They periodically publish the current hash chain value in a newspaper or other offline redundant record.
You don’t have to send your private documents/transaction to the notary for them to be time stamped and recorded. The notary is just certifying that something that matched this hash existed at this point in time.