a bit of a bind huh!
heat/ice could damage the frame paintowork & material heat-treatment, so it depends on how you feel about that, but thermal expansion differences should generate a clearance
not sure if this will work, but how about putting an aheadset star-thing into the seatpost and using that (with the frame in a vice - gently with a rag) to pull against with a long bolt through a plate ontop of the vice? (make sense?)
you also might find that overnight the WD40 has soaked in a bit loosening it
otherwise you've got a nice shim already inserted into your bike! (or take to a small engineering machinist and they'll machine it out for you)