[prepend]

The hashing needs to be done by a trusted third party. It would be a cheaper to operate service than wayback, but would let you check individuals content against manipulation.

You have to incentivize people running and storing the hashes.

[Buge]

You could put it in the bitcoin blockchain. Or if you don't need that level of complexity and cost, you could put it on twitter, which doesn't allow editing tweets (but does allow deleting).