The seat shown has plenty of wood, appears more sturdy than many -- sturdy enough as is to carry even an overweight paddler. To provide any real new strength, the splines would have to be quite precisely fitted, and I think the minor, unnecessary gain in strength from splines would not be worth the effort involved -- a lot of needless work in this case, IMO. Indeed, even if you put in splines, it might look better if the seat frame were inverted. The rails all appear to be simple rectangles in section, with no bull-nosing or other edge treatment, which might argue against inversion. The seat itself is rectangular, not trapezoidal, and if flipped side to side rather than front to back, the miter-cut ends of the seat rails should just fit back in nicely -- short rail stays nearer the stem and the longer rail nearer the center of the canoe. Unless the end miters have been cut with a double angle to match both the fore-aft taper of the hull and the vertical angle possibly induce by tumblehome (unlikely, since the hull appears to be essentially vertical at the level the seat is mounted), they should fit well. However, the ends of the rails and the bolt/screw holes on one side look like they might need some attention -- a better place to spend time and energy if that is the case.
