Maybe just publish a sha512 hash of the document in a newspaper classifieds section... If the goal was to have recorded evidence for posterity.
I wonder what the odds are that the current block-chains will be active in 70 years? Probably lower than the likelihood that all public tweets are recorded in the library of congress or some similar institution.
Does a 60 year old archive of a block-chain that "lived" for 15 years of any value? It could probably be "faked" with a cluster of machines and some time?
At any rate for things like a paper, we already have the ability to go and get a document notarized somewhere -- and that has much more legal precedent than some clever mathematical hack.
Would be interesting to see law firms offer "adding to and helping maintain" a block chain as a supplement to regular notary service though.
[ed: Ah, contributing to the slashdotifization of hn. I see that the article is about a company that tries to do this. I still wonder about what happens if/when "everyone" moves away from (the current) bitcoin (chain) to a new protocol. I suppose it could be "easy" to log the current state of the old chain, in a new chain. Would make verification more complicated though?].