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This is not a question, just writing up what happened for posterity and for Google.
I have a carbon post in a carbon frame with an alu shim. It was fine for years, but started to get stiffer more regularly. Presuably the anodising wore off, so eventually it became jammed solid as I didn't get it out to clean it early enough.
I read up and decided to try caustic soda. Applied a little bit of concentrated solution and got some fizzing, but after a few hours of regular applications got nothing. Screwdriver in the shim split to splay it slightly followed by penetrating oil didn't do much either.
In the end, since it was an aluminium shim I tried heating it up on the basis that the shim would expand, splay the seat tube ever so slightly and release the pressure - which released it just enough to allow removal with a huge amount of force, grunting and swearing.
The point of the thread though is about caustic soda. I tried to find the reaction that dissolved the aluminium oxide, but all I found was loads of hits about aluminium itself reacting with the caustic soda. So I tested it out and put a bit of aluminium oxide in my solution and - nothing. However aluminium did fizz. So the fizzing seems to be the alu itself being dissolved. This might help release a post all the same, but it'll probably bugger up any of the alu components so don't try it with an alu frame.
I used Waxoyl on the shim to help prevent corrosion. Excess causes seatpost slippage, so keep wiping it off.
PS anyone notice how Waxoyl is basically the exact same stuff as dubbin aka Nikwax for boots?