Thanks for the link. There isn't much of the body sticking out of the head, and I don't know if theres room for a dremel tool - but you got me thinking. Could use a screwdriver inside, would have to maybe hammer it down, and by doing that risking to loosen the glow plug tip, but could then use a wrench on the screwdriver to force it. Maybe...
A slightly better idea may be to pick a Torx bit thats a tight fit and to hammer that in, gives you a better grip on the glowplug body ( six points of contact instead of two ) and better leverage to turn it with a ratchet handle or similar.