Just as a general point.... if I wanted to protect the hash from Quirrell modifying it, I'd have stored it as a private prediction at predictionbook, and possibly on a blog or two.
A general solution would be something like my archiving setup, which resulted in this page being crawled by the Internet Archive in early 2012: https://web.archive.org/web/*/http://lesswrong.com/r/discussion/lw/421/a_sealed_prediction/
I would like to make a prediction, but I'm not ready to raise the subject of the prediction just yet. So to show that it was not just obvious in hindsight, and to strengthen my precommitment to write an article about it later, here is the sha1sum of my prediction: 9805a0c7bf3690db25e5753e128085c4191f0114.