I've had a couple of stuck handlebar stems lately and I've found that I can usually get the wedge out by taking the stem out rethreading the bolt back into the wedge. Then I put a body puller slide hammer on the head of the bolt and give it a couple of good shocks with the hammer and the wedge breaks loose and slides out. Use a brass brush wheel on grinder motor and brush the wedge till all the rust is gone. Same method for getting rust off a seat post. Breaking the rust bond on a seat post can be overcome with my 18" pipe wrench.
If you can get the top part of the post out of the seat tube, you might be able to retread the bolt into the wedge and figure out a way to apply some quick shock force to it and break it the post loose. Steady pressure doesn't work, it needs to be shocked loose.