I spent the evening in the garage after making my post. I figured out what's going on.
The electrode is fired in the ceramic, the ceramic is swaged into the threaded body, and finally the plug terminal is screwed and glued into the top of the ceramic. The top of the electrode is sized to just fit in a hole on the bottom of the plug terminal. The tolerance between the electrode and and the plug terminal has to be tight but lose enough to allow assembly.
I removed the plug terminal from four of my high-resistance plugs, did what I could to clean the bits, then placed a few strands of fine copper wire into the hole of the plug terminal, and screwed it back together. Measurements showed that the resistances had dropped from mega-ohms to nothing. I am not proposing fixing plugs... only suggesting that the high-resistance I was measuring was largely due to the assembly method and perhaps some minor internal arcing between the electrode and the plug terminal. The film thickness and clearance was obviously less than the spark plug gap so the plugs were functioning normally.
Because of what I saw with the plugs I took apart (Champion and NGK) I don't think I would put any faith in pass/fail resistance measurements. I think the only real test would be to look at the color of the spark that a given plug can generate.
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