I've had bikes with seat posts in this predicament many times. Kroil and a slide hammer usually works well enough. I've built myself cutting chisels out of naval bronze rod. It'll part the aluminum seatpost and do no harm to the steel tubing. Just don't get over-zealous with your hammering, or the bronze rod will deform the seat tube from the inside. That will ruin your day, and don't ask me how I know. I have never tried the use of caustic soda to remove the seatpost. Youtube is a good source if you want to know how to do it. I'm just ticklish about the idea of putting that stuff inside my frame, and prefer mechanical methods.