Try several cycles of heating the post with a propane torch and letting it cool completely. Use some care to not let too much heat transfer to the steel frame as aluminum conducts heat pretty fast. Maybe keep your hand on the seat tube and when it gets warm stop heating the post. Soak it good in penetrating oil between cycles. Wipe off the excess before reheating-don't need any flames. After several heat-cool cycles and some good soak time get your pipe wrench out and see if you can break it loose. Might try cutting off the post and whacking it with a hammer a bit driving it a bit further into the frame as well-your goal being only to break it loose.
BTW my favorite penetrating oil is a John Deere branded aerosol that I picked up at a local Deere farm equipment shop. That stuff seems to really work. Kroil is another favorite.