Option 1. You can use LoyceV bot for the job by sending it a PM and make sure that you set the delay for publishing/revealing if you don't want the default of 7 days. The drawback is that LoyceV can see the original message from second 1.

Option 2. You do it yourself. Make the message, sign it, publish the signature and the address. When the time comes, write another post (don't edit the first), and this time post the full message, the address and the signature.
People will be able to compare the signature with the initial one and they can also verify the message; if it's the original message indeed, the signature will be verified.