I had to remove a stuck gearbox shaft that was stuck in its bearing and know amount of gentle persuasion with blocks of wood and heavy hammer worked.
Finally i tried a hot air gun [electic paint stripper type] Heated up the whole of the shaft and bearing. Then when it was all nice and hot poured cold oil on the shaft, causing it to shrink at a different speed to the bearing. One swift blow with the hammer and wood block and the shaft came free.
If you can warm the engine up to temperature as you suggested and then carefully cool the offending plug you may be able to free it in a similar manner Maybe a cold plug socket/ wrench will be enough to drop the plug temperature quickly and cause the plug to " shrink" enough to free it. As stated the use of a hand held ratchet bar to feel the plug as it comes loose.
