If you can prove the existence of the encrypted thing before some point in time than you could prove the existence of the unencrypted thing before some point in time.

There isn't any way to do this without one or more trusted third parties. Traditionally that would involve someone like a public notary or a lawyer.

I was amused to find that there is a service that cryptographically timestamps things over email via PGP that has been running since 1995:

* https://www.itconsult.co.uk/stamper/stampinf.htm