"[...] into the future, or at least the most likely future."

If less likely futures are game, then this is solvable.

Encrypt the message and destroy the key.

At the press of a button, generate a random key from true random numbers, and try to decrypt with it.

The seer will be able to see the vanishingly unlikely future in which the random key guess happens to match the key that was used to encrypt.

"[...] into the future, or at least the most likely future."

If less likely futures are game, then this is solvable.

Encrypt the message and destroy the key.

At the press of a button, generate a random key from true random numbers, and try to decrypt with it. Show an error message if it fails, but do not display the key.

The seer will be able to see the vanishingly unlikely future in which the random key guess happens to match the key that was used to encrypt.

You could explain it away as saying all the futures where it doesn't get properly decoded "collapse" into one since the outcomes are identical, so it's easier for the precog to see the one where it does get decoded.