Ken Freeman offered the kernel of another idea: using a brake cyclinder hone to grind out the aluminum post. Although, as he suggests, some coarse emergy paper might work, the hones are almost certainly going to completely clog with aluminum within seconds. I haven't tried this with hones, but they seem to be carborundum or equivalent, just like grinding wheels, and I have had the privilege of dressing grinding wheels that someone had used to alter the shape of an aluminum piece.
But, I'm posing a theoretical objection in the case of hones, and someone might have had real empirical experience.
harvey sachs mcLean va
Here's another idea. For automotive brake cylinder repair, there's a tool called a brake cylinder hone. It has a few cutting stones on little arms that are spring loaded to press outwards. This should (might?) fit into the seat post ID. Then it gets chucked in a drill or maybe a Moto-tool and you spin it forever to cut out the aluminum from the inside. One problem is that if you stick it in the seat tube too far and it goes past the bottom of the stuck seat tube, you might not be able to get it out.
An alternative that uses heavy emery cloth or paper abrasive might be rigged, that would at least pull out if you stuck it in too far.
Maybe another list member knows of a more elegant tool for trying this out?
This "method of attrition" would at least give you an alternative to more corrosives, at least something different to do with your time.
My heart goes out to you - there but for the grace of God go I!!
Ken Freeman
Ann Arbor, MI