Cousin Sailormon:
That's an interesting design concept. The core question is whether the flex experienced with the standard design is, in fact, a problem. There will always be some flex in a boat, even with your stiffening modification, so how much is acceptable? Some structures need to be quite rigid to fulfill their design requirements. Others require more flexibility. What degree of flexibility is optimum for a kayak? It is likely that flexibility improves hydrodynamics up to some point.
To me, it feels a bit like a solution looking for a problem. I built and paddle a Ravenswood, which is 15.5 feet, not one of Jeff's longer boats. I can feel it flex, especially when there is chop of a certain size, but it doesn't constitute any sort of problem. If I were building a longer boat, I would pay attention to the type of the wood I used (both species and individual sticks), scantlings, and the grain (straight with minimum runouts).
Fair winds!