I used a LOT of high quality penetrating oil (Penphite in my case) over a period of about 10 days, letting it flow 'down' the posts. Using the rubber mallet Scott mentioned worked for one of the headrests, and one side of the other. BUT even after another 5-6 days of applying Penephite, the 2nd one on the 2nd seat would not release. I had a spare headrest from another seat, so I removed the small vinyl cover on the side that wouldn't release, then used a cutting tool to cut the horizontal piece where the small vinyl cover had been. Removed the headrest and the one post that was free, then used large water pump pliers to twist the frozen post. It did come free and I removed it.
I don't recommend this method unless you have spare headrest hardware!!! By the way, the older seats and maybe most European seats have 10 mm posts. My '74 has 12 mm posts. I do have a spare 10 mm headrest hardware assembly if you or anyone needs one.
Gary