A few comments- The bonding of the steerer and the headset parts is at the upper stack, not the lower one (or I'll eat my hat). WD40 is a poor penetrant. Koil, ATF, PB Blaster all are far better. But just having a penrtrant get past the corrosion does not remove the mechanical clamping of the plug (understand that most corrosion is a larger molecule, rust adds dimension to the surface). Hence the suggestion of shock to break the structure of the corrosion. Since the fork has some free play within the bearing perhaps a downward impact to the upper cup is in order. Place a tireless wheel in the fork (do not just place the drop outs on the floor!) and a length of solid pipe over the steerer that also contacts the upper cup then whack away. Andy