TT - yup..it was indeed a membrane that we based our calculations on. very astute! i've admired your posts ever since i got on here for their clarity and depth of explanations. you know your sh*t. nonetheless, i still disagree in the direction of expansion of the hole even in this case. i base this not only on a measly calculation from years ago but some practical experience as well. for example, heating manifolds to withdraw bolts, pump casings of rather large sizes and casting complexity (nowhere near our precious honda heads though) In large diesel engines heat is often used to enlarge the cylinder wall to slip liners out of cylinder borings and one doesn't go around heating the entire body of a diesel engine. in fact, it's probably one of the few ways to get one out. sticking to practical applications i have yet to see anyone apply 'cold' to a casting to slip a stuck thread but then again, i certainly am open to another technique. it's certainly common to toss cylinder liners in a freezer prior to slipping them into place.
so let's consider your cooling idea. it has merit. there was another thread on here for removing dents using a slight amount of heat and some liquid CO2. perhaps spraying the plug with the liquid CO2 would shrink the plug and break those jammed threads?? certainly less obtrusive than blasting an aluminum head with heat. I noticed that the key to the CO2 was to hold the can upside down so that the liquid came out instead of gas. the liquid has MUCH more cooling properties than a gas (change of phase and all that). certainly worth a try if the liquid could be applied to just the plug body.
like you, i've never had much faith in 'easy'outs. heat has been my friend. and you can believe that i will now and forever more apply anti-seize to my spark plug threads.
frank