I would use a "Set Back Seat Post" and just reverse it. A long time ago I saw a Quill Stem that had been modified into a seat post. I cant find a picture of an example but its a thought.

Personally I would think of an aluminum seat post as a piece of heavy duty aluminum conduit. With that thought I would go over to an Electrician's Shop and have them put a 20° bend in my conduit (seat post). I also remember that in the Army the guys in the Motor Pool would heat up just about all metals to at least 300°F before bending. They said that you really never knew what some of their alloys were made of and getting the metal a little hot always helped. Of course that was the 70's. Who knows what alloys are used today. I dont think I would put fire on a magnesium alloy part for sure. Ha...