No guarantees, but if you can't get the warp out by loosening and regluing some glue joints or misting the wood, I have pulled similar warps out of wings while covering the plane.

Cover the wing as usual.
After the covering is shrunk down evenly, you may need a helper.
Have your helper (or do it yourself), twist the wing to slightly past straight. This will cause wrinkles to form in the covering. Use a heat gun, and re-shrink the covering to shrink the wrinkles out, and if you worked the wing far enough, the re-shrunk covering will probably hold the wing straight.

I've never built a large plane like yours, but I have worked warps out of wings on smaller planes using this method.