Liquid wrench works better than wd-40 for rust penetration. If that fails, you can torch it, and the rust will break down as the metal expands. Be careful with the flame, as prolonged exposure to the porcelain will "blue" the finish. You can also try weakening the nut with dremel cuts and hope you get a clean break from the thread when it snaps. I would bet that the threads on the legs are done for though, but you can always try to clean up the remaining threads with a die. Good luck!