Try approaching things from the other side. Get a washer whose overall diameter is slightly larger than the hole on the bottom side of the head tube but will fit between the forks and also has a centre hole which the stem bolt will fit through. You might have to use a pair of washers to get there.

As others have suggested use some releasing fluid. Screw the stem bolt from the other/bottom side into the stem wedge and then tighten it up to pull the stem wedge out. I assume you are able to remove the handle bars. As you tighten up the bolt periodically tap the stem wedge around its circumference from the top using a screwdriver and hammer.

I have had success doing this in the past and kind of surmise that what has happened is the the stem wedge has been rotated vertically and ended up being lodged in the steerer.