First I want to tell you that I have worked with chemicals and aluminum for probably 10 years professionally. Not many acids will impact aluminum so forget about sulfuric, nitric, hydrochloric or any fancy acid. If you don't know what it will do to the frame metal and finish then avoid it.
Now aluminum does react with bases and sodium hydroxide in particular will actively eat away at aluminum and not touch steel. I know that not many people have sodium hydroxide just laying around the house but you may have something very close to it. Baking soda is a base that will attack aluminum and not steel. You can try a solution of baking soda and water for a few days and see what happens. Also, I do not believe that there is much acid in coke. It does contain however sodium bicarbonate another base which will attack aluminum but the soda it not concentrated enough to have an impact. Besides it will get real gooey when the sugar starts drying unless you use a diet brand. You can try the baking soda route but I still believe some good lightweight lube and force will work but if all else fails then try the baking soda trick. I personally never tried it on a seat post but I have etched hundreds of pounds of aluminum in my past and can vouch for it. If you drop a hunk of aluminum in some concentrated sodium hydroxide it will fizz like bromo or alka seltzer (other bases).
Ray Homiski
Elizabeth, NJ