Oh, I should have added that I've thought up at least one solution that might work, and I'd like to hear some opinions on it.
Most of the forces from the rig will be acting on one of the gunwales and the "keel"/stem/stern assembly. As someone who has also built a few bows, my first instinct is to use thin (3mm or 6mm) okoume marine ply, a flexible bowyer's epoxy, and some light (2oz) fiberglass cloth to build up those members of the frame out of a fiberglass/okoume sandwich, similarly to how you'd do it with an exceptionally heavy bow, then clean them up and attach them to the boat as normal. That way they remain flexible, but much stiffer than before, and the composite is less likely to crack under the strain of a hard bend as the hull digs into a wave at 15kt than solid milled wood is.
My other thought was to use riven wood instead of milled, but that would involve finding a tree and splitting out my frame components myself, then processing them down to the correct dimensions without violating any growth rings. It would be much more "traditional" approach, but also much more labor intensive and probably less consistent than the composite method.