You're safeguarding against the wrong thing. If I needed to fake a prediction that badly, I'd find a security hole in Less Wrong with which to edit all your comments. I wouldn't waste time establishing karma for sockpuppets to post editable hashes to deter others from posting hashes themselves, that would be silly. But as it happens, I'm not planning to edit this hash, and doing that wouldn't have been a viable strategy in the first place.
"Clearly, the way to make our safeguards super-secure is to make yet another comment with the hash."
"Clearly, the way to make my safeguards super-secure is to make yet another Horcrux."
Somehow, you could only see through one of these strategies.
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.