[jerf]

"One solution would be to encode the prophecy into a blockchain, and reference that in the envelope. This way there's multiple witnesses that you did something that day at least."

That's the basic scheme. Variants on this idea have been in use for centuries. Modern hash schemes, including those used in the blockchains, make it trivial, albeit at the price of byte-for-byte precision; you just have to provide reasonable evidence that something existed at some point in time, which is easy with twitter, facebook, google, and literally thousands of other places you can stick arbitrary content that will end up with a timestamp that you can reasonably claim you have no ability to forge.