Some very useful science there from the late lamented Sheldon, but one thing he didn't mention that I thought was a recommended last resort is to drill a hole horizontally through the post just above the insertion, the hole big enough to run a decent size long bolt through it so you can then use that to lever against the frame. If you clamp both ends of that bolt onto a workbench and turn the frame against it, that's the most leverage you're going to get.
Perhaps do that in conjunction with Sheldon's advice about aluminium shrinking more than steel at low temperatures - stick the frame in somebody's chest freezer if you can, then a freezing spray down the post, then maximum leverage. Sounds like good fun!