I think you need part of a laminated ring frame here, Scott. A full ring frame, that is as nearly a circle as possible, joins the partners, with a force acting in one direction, to the step, with a force acting in the opposite direction, elegantly and economically in terms of strength/weight. If the hull is not strong, the full ring frame is desirable; but your boat is likely to have a thick, strong hull construction, so the lower part of the "ring" is already there. You just have to add the upper part, not following the profile of the coachroof and deck exactly, but bridging across the corners. You're on the right lines, in mentioning hanging knees, and I think that you just need to combine four hanging knees into one laminated beam that spans the boat from side to side, linking the partners to the hull.
This is what I've done in a similar scenario with Weaverbird, using 12mm plywood. You could do the same, using 18mm plywood, but laminating a beam that has a similar shape to the curved bottom edge of my beam would give you the more open look that you're aiming at.