Eric,
I think the daggerboard-style rudder in a cassette sounds like a good idea, but a couple of questions/problems would be: How can it kick back if you hit something? And how much weight would it add to the transom?

The new Farrier 33 has this style of rudder, and it also has an optional kickback rudder sleeve. But on that boat the kickback feature can work because the transom is angled back. I don't know this could be made to work on a plumb transom. It's interesting to think about, though.