try warming up the pipe where the stem goes in and not the stem. same principle as getting a stuck nut off of a bolt with a torch, just dont put too much heat on it. holding some ice on the stem may help the stem from expanding as you warm the pipe up.
if nothing else works, just throw it in the tub :lol:
dont know if this would work or not or possibly leave residual taste, but try letting it soak in a mild soapy water solution (you already know how to do that :laughat
or some olive or veggie oil which may be able to penetrate as like when you use a penetrating oil, wd40, jb80, powerblaster on a rusted bolt.
if anyone thinks this may be bad for the pipe please say it so ace doesnt damage his pipe!