Jon, just to safisfy yourself glue some panels together in various configurations of tape,wood,and epoxy and see how they hold up. As Paul has said the glassed joints are stronger than the plywood, I don't think s&g joints fail much up in those areas that you are speaking of. The stresses are greatest where the stresses are being applied, in other words the paddler sitting on the kayak, or elbow in the deck. Sounds to me you are looking for something "better". The bottom/side joint is already a strong joint with epoxy and 6oz glass, if you are looking for "strong" then look to where strength is needed first. If you are looking for "light" then look at what uses the most epoxy first and reduce it. I could see exotic fabrics having a place with 3mm plywood but simply using lighter glass cloth and using more of it in the places where it's needed will maximize the strength/weight/cost issue.

Here's anothe stab at the concept: for a joint to fail because it had 6oz of glass instead of 5oz of carbon/kevlar the adjacent flat panel area has long since delaminated.