Interesting, but the PRF / LargeBlob extensions already enable just such functionality (and more) without relying on the secrecy of a public key.

Why not just use those?

Edit: that's what I get for not reading far enough -- the article addresses this, though I would quibble with the confident assertion that the extensions are not available in major browsers, given I worked for a startup literal years ago which built major functionality on top of these extensions, which were available in (at least) all relevant mobile browsers.