removing shaft of alu quill stem from aluminium fork.

I looked into this quite a bit for a seatpost and a stem I was trying to avoid cutting out of steel frames recently.

Aluminium oxide is described as amphoteric which means it may be susceptible to acid or base so vinegar, coke, lemon juice are all possible treatments. Caustic soda will eat your fork as well, as you expect. But all aluminium has a layer of oxide on it, and an aluminium fork in an aluminium frame should not corrode further since there is no galvanic potential between them - this problem of corrosion and sticking is for aluminium components in steel frames.

Your seized stem should therefore be due to friction from a very close fit or maybe dirt and old grease. You could try heating the steerer and hope that because it is on the outside, it heats up and expands more quickly than the stem, freeing it. You could try cooling the end of the stem at the same time (e.g. stick the stem end in water/ice/salt mixture while heating the steerer). If dirt and grease is the problem, a penetrating oil (plusgas?) seems most promising to me, but maybe coke (I'm not convinced that it will work its way in very effectively).

Also, if a friction problem, I read a really nice thing about somebody trying to get a carbon seatpost out of a carbon frame. He reasoned it wasn't corroded, but was stuck by friction, and that pulling on the frame and the post served only to stretch and narrow the seat tube, gripping the post more tightly. He created some sort of system that held the frame and pulled the post and it slid out. Don't have a link for that though, sorry.

Gripping the stem end with the strongest vise grips/biggest wrench and trying to get it to turn is also a good thing to try from time to time!