Ok - I sanded a little groove in the deck/concave side and epoxied two bands of carbon fiber tow. Blocks under, and sand bags on top. Had the curve set just how I wanted it.
Bottom line: (-) Had some uneven "spring back" that's not what I was after. (+) Nice and stiff.
Since I was after saving a little foam, and minimal hotwiring, and since it didn't come out close enough the first time, I'm taking the (+) away, and not trying to figure out how to get it "just right." Getting it just right is more work than just hotwiring.
The utility of the stiffness will be: I'll shape the bottom, put a nice little stiff smooth, flat strip in the tail section, shape and glass the deck - 'cuz I like to glass the bottom nice and smooth over the lap, and then I can be as precious or minimalist as I want on the deck side lap