One man's Mede is another man's Persian.
Have you considered a PM to Philip Morgan (philipmorg)?
Rumor has it that an even better penetrant than PB Blaster is a 50:50 mixture of acetone and ATF (automatic transmission fluid). I haven't had the opportunity to try it myself but know some car mechanics who really like it. Acetone is flammable and moderately nasty, so I wouldn't use this indoors without significant ventilation.
This problem sounds remarkably (or not) similar to the common problem in older bikes of the aluminum stem getting stuck in the steel steerer tube of the frame. Google on that issue and you will find all kinds of tips. Boiling water is supposed to be fairly effective, I have an old bike with a stuck stem expander bolt I'm going to try that trick on soon..
Being a mechanic by trade, (no longer working as one) and living on a farm has given me the opportunity to be forced to remove all kinds of stuck parts, and liquid propane has been a staple for many years.
I like the idea of the freezer and boiling water, it might just work.
The problem and solution are fairly simple, use temperature and expansion rates of dissimilar metals to increase the gap between them and allow them to move freely, unfortunately implementing of the solution is not always as simple as the solution itself.
You most likely have a small amount of galvanic corrosion that is not allowing the parts to be separated. Penetrating oils will help but by far the most effective method is to make the inside part smaller or the outside part bigger.