Ken,
There are a few different things going on with a sailplane wing compared to a stunter. First, the flex helps absorb the surging force at initial release to keep that peak force down. And once the climb angle is established, you can modulate the winch to keep the loading below the limit load. Second, the wing in climb flexes and reduces the projected area and therefore the total lift and thus the maximum moment load at the wing root. Finally, once the line is nearly overhead, energy that was stored in the deflected wing enables a zoom launch, not just coasting upwards while bleeding off excess airspeed. Think of the wing as a deflected spring with energy stored in it like a diving board. At least that's how I see it.
If you draw a free body diagram of a stunt wing going thru a hard corner, you should find that the entire weight of the suspended fuselage (less any body lift of the fuselage itself) causes a bending moment in the center of the wing. Making the wing more flexible doesn't change that. To get the total lift, integrate the area under the lift distribution. To get the total moment load on the wing root integrate the lift distribution as a function of semi-span with the origin at the center of the fuselage.
The two Nobler ARFs that I have cut into had a single layer of 1/16" center planking.