Hi there,
You are in trouble, and you will most likely have to remove the head to fix it. The way to remove it is to insert a bolt slightly smaller in diameter than the hole and carefully mig weld it to the broken plug. The plug housing will shrink as it cools and you can easily remove it by using a wrench on the bolt. As I said, this will require removal of the head, but I have used this trick on both sparking plugs and broken studs, and it does work very well. On a broken stud you usually have to just build up the end of the stud about 1/4" with weld before you can attach anything to it.
Good luck!
Richard Wooldridge
'82 300D/4.3L V6
'74 280C
'89 Astrovan
'85 GMC S15
'77 Jag XJ6
'77 280Z
'94 3.8 Taurus
None have ever been to a dealer, and everything works on all of them...