Surely you would have to store something off-chain in order for it to be inaccessible until a given point in time.

Unless you can predict the future, I'm not sure how you would generate a key that would be unknowable now but generally available in the future.

You might be on to something there.

I was thinking of encrypting a secret in the structure of a Rust program so it can only be decrypted by compiling and running it.

Can’t you just fork the chain?