[Cryptography] Came up with a weird use case, got questions

[Joshua Marpet]

My mom passed in 2013. I had a nice day, where my wife, daughter, and
myself, went to an interesting and fun wedding today. I had a random
thought, that I would have loved to talk to my mom, and tell her about it.

Obviously, not possible, short of a OUIJA board. :)  So I thought about
writing a letter. So where do I post it?

So I thought of a website, where you can send letters to people, a la
Postsecret. But it's harder to anonymize electronic  messages.

Soooo, how about a system that automatically encrypts incoming emails? And
then, some time later, it decrypts, long after anyone who wrote those
letters is alive? Say 100 years. I'd write a letter to my mom which would
be auto-decrypted 100 years later and given to the public. A la Post
secret, and historical interest.

Any systems out there which will auto-decrypt, not based on a clock (which
can be spoofed), but instead based on an event, a piece of information, a
trusted info source? Something like the Long Now foundation's clock?

Weird use case, I apologize. Just a thought.
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