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Mike-
In situations like this, when two very similar metals make up the stuck 
joint, I have had much better success when the cooling medium can contact as much 
surface area as possible all at once.
Dry ice can only really transfer its chilling effect where it touches, and 
even if it is crushed up into small pieces, that is still not much more than 40% 
of the surface area.
However dry ice and methanol work very well together, since methanol stays 
liquid at the -120f or so of the dry ice.
We make up a batch (when the solid CO2 stops boiling violently and settles 
down to just a "simmer" the methanol is thoroughly chilled) and then work with 
it.
In your situation, of course, the bottom of the cylinder sleeve needs to be 
plugged to keep the liquid in place. 
I have solved that need in the past by using "Great Stuff" spray foam (the 
more dense version ) to create a dam at the bottom, let it cure for about 30 
minutes, then quickly poured in the methanol/dry ice mixture. I don't know how 
long the foam seal would have held because in about 20-30 seconds, the insert 
broke loose and the tension of the puller on the stud which passed through the 
foam dam was released, breaking the foam dam.
But since the insert was now loose and about 2" out, it was a moot point.
The rest of the extraction was a breeze. 

-Bruce Thompson