Won't you find it hard to steer with the blades pivoted up even a small amount? Why not just leave the lock down rod out of the tiller arm hole and strap it along the arm with a bungie or velcro? If am inpact occurs, the rudder will put up quite easily.
BTW, I've hit a few marine sunfish (~3'x3', slow moving, surface loving beast) doing 20kts on my Tornado. The AHPC taipan rudders popped up beautifully everytime, no damage to blade or release system (but I think the fish didn't fair quite so well ). I really liked that setup...simple, reliable, low cost, light. My new boat came with the Marstrom system stocks. While thay also use a lock down arm, there are a lot of little springs/rollers involved and they do not hold up at all when you hit something. Might try to modify the stock to be more like the AHPC's.

Mike.