A practical idea. I wonder whether a more consensus-based alternative to this could use some aspects of the current-highest-block from a blockchain to incorporate proof of a previously-unknowable, globally-verifiable value.
Some notary services allow you to inject documents or hashes into cryptocurrency blockchains which prove that the submitted documents existed before a timepoint.
Not with what GP proposes.
If I understand their proposal correctly, it is to include a value from "the current-highest-block from a blockchain to incorporate proof of a previously-unknowable, globally-verifiable value." You can always include historic values trivially.
It is much more involved to actually do a transaction on that chain, which could indeed be used to prove existence before a certain time. But yes, your alternative proposal would work for that, if you assume the chain cannot be controlled by any individual/partisan party.