Some progress (not really).
Fold, and soak, roll tightly and let dry. When unrolled you get these kind of bulges along the "cloth" seams. However, as soon as you curve the overall shape of the sail in the other direction they mostly flatten out.
I think one would have to also curve the roll while it dries to get it to settle with both curves. Maybe roll it up on a thick metal wire and bend that without somehow crinkling.
Square sails also bend inward along the corners where the ropes attach.