That was one of the first things I tried. I used penetrating oil rather than WD40, couldn't get a whole lot in there though, since I didn't want to risk having any drip onto the paint on the back of the dial glass.
I actually made a sort of jig for pulling on the knob... took a few inch long section of 1/8" x 3/4" aluminum flat stock, cut a notch in the middle to fit around the shaft, then put a couple holes in the ends and looped some twine through those. I slipped that behind the knob and pulled on the twine. I succeeded in bending the aluminum, but the knob wouldn't budge.
