The question was dynamics of the existing rudder system? not really alternatives. srm, I was thinking more bolt on extensions, using the existing pressure points. The added beam may be too much for the 20>
I think the rudder system should be the least of your concerns.
I'd be very interested to see how are you plan to bolt extensions to the crossbars. Are you going to add these extensions to the ends of the crossbars, or cut the crossbars in half and add the extension in the center? Some seriously high loads there and obviously a critical element to the structure of the boat. What are you going to use, aluminum tubing?
A 10 foot beam Hobie 20 sounds like an awesome boat, but honestly, I don't see how you're going to extend the beams. I think the only way to do it would be to use beams that are already the correct length.