Sequential delayed release of chained HMAC keys for use as asymmetric signature Iterative hashing of a private random seed can yield keys for use in HMAC, which if revealed publicly in reverse order and on predetermined schedule, can be used in a manner similar to other asymmetric key signing protocols, where released intermediate key authenticity is verifiable in terms of its relationship to the first released (last in chain) key. The seed is like a private key, and the last of the chain (first released) is like the public key. Useful elaborations are possible, e.g. converting the chain into a branched hierarchy for delegation purposes.

This can't be a new idea. Is it? Does anyone know of existing work on this, either in the journals or in the wild?

This link restates the above with more detail, some diagrams, and a few elaborations:
http://ben.mord.io/p/delayed-chained-key-revelation-dckr.html
That sounds like Tesla, which also uses a chain of MACs (where each key can be computed from values later in the chain), revealing the previous MAC key with each packet.  Going through the paper, I don't see how they authenticate the last value; my recollection was that they used a conventional signature scheme.