@emily Given that this is about information I'm pretty sure you cannot get such a hard proof, because entropy famously only works one way.

For a softer proof you can publish the thing (or a cryptographic hash of it, if it shouldn't be immediately public) and ensure it is widely circulated and referred to -- this is kinda the converse of the approach you described, or at least uses the same ordering.

Interestingly (?) blockchains implicitly use this approach.