You might try soaking the wood in ammonia and then hold it in the shape you want. When I was a graduate student in the Wood Science department at Colorado State, we would take 1/8 in thick oak strips and soak in liquid ammonia (under an exhaust hood in the lab...don't try this, the vapors are poisonous!) and we could tie the wood in knots. After the ammonia vaporized, the wood would stay in that shape. Ammonia plasticizes the lignin in the wood. Lignin is what holds the wood cells together, and is what is dissolved out of wood chips to make pulp for paper.

What I can't tell you is how much lignin is in balsa wood. it is a tropical hardwood and different anatomically than our domestic woods, but I suspect that any solvent with ammonia in it, like windex, would help bend the wood. The more ammonia the better. In fact, you might just try straight ammonia. I have used windex to spray on sheet balsa as I formed the front of a wing and it helps keep the curvature.

Give it a try,

Terry Brown
Wood Science and Engineering Dept.
Oregon State University