I recently used Corrosion Block to help free a stuck aluminum alloy seatpost from an aluminum Cannondale - it still required clamping the SR Laprade seatpost top in a bench vise and I gladly paid the strapping young LBS owner $20 to muscle it loose.
I used the same product to free the stuck stem in my Sirrus; that I was able to muscle free.
I recently also used a mix of Corrosion Block and Liquid Wrench over several days to free another stuck seatpost. That one required a rubber mallet to the saddle nose to break it free so it would start turning.
So long as the seatpost heads are an integral part of the post the bench vise works wonders.