I've always wondered: has there been any effort to implement a PoW challenge like that at a lower level? I.e., TCP but the handshake requires solving a challenge, otherwise the connection is just closed? It seems like something that could benefit from being invisible on the application layer.

Edit: To answer my own question, yes: http://www.arijuels.com/wp-content/uploads/2013/09/JB99.pdf

Edit 2: Maybe TLS would be another reasonable place for it?