Has anyone tried a mechanic's air chisel, like you find at a muffler shop?

Turn the frame upside down and fill some penetrating oil into the seat tube. Use an air chisel with a blunt pointed tip and gouge into the seat post in the direction of removal. The few thousand vibration-hits a minute from the air chisel should work the penetrating oil between the post and the frame and also work to break the corrosion between the post and frame. This is how we'd remove rusted together exhaust parts when one needed to be saved.