That's an interesting idea. Fortunately in my case I didn't have to use a nut after all. Thinking back over the whole job, I was able to back all of the glow plugs out the first day (while the engine was still somewhat warm) to the point that all threads on the plugs were out of the head. In retrospect I think that if you can get to that point, you may just have the problem licked. The reason I say that is because it's then easier for the penetrating oil to seep past the top of the glow plug and down into the head, softening up the carbon which binds the plug. (I had also used Maxbumpo's tip of removing the injectors and spraying the prechambers from inside.)

I had been hitting those last two stuck plugs a couple times a day with the PBB, and when I went back Sunday afternoon to tackle the last one (#6), I put a wrench on it and was surprised to find that it turned easily, with no cracking or drama. So by using a pair of cutting pliers to grab the notch near the top of the plug and leveraging against the engine, I was able to pull the plug the rest of the way out fairly easily.

On reaming the holes for the #1 and #6 plugs, I found the reamer came out covered in black goo, vs. the usual powdery state of the carbon. So I figured that giving the penetrating oil a few days to do its work was very helpful here.

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1995 E300 200k
1981 300GD unknown km