> Hashing the data that's added to the archive and then putting the hash in the blockchain would reasonably prove the data in the archive hasn't been modified at a later date.
Unfortunately, the files themselves aren't public, and each file contains dumps from hundreds of websites, so even if they were public they're not the easiest thing to verify.
Still, being the guy behind OpenTimestamps I should point out that in this case I don't think timestamp proofs really add that much: Reid's claims seem dubious even without cryptographic proof.
Here have yourself a timestamp https://petertodd.org/2016/opentimestamps-announcement
https://github.com/opentimestamps/opentimestamps-client