Option 1: If you remove the head you could use "nitric acid" to dissolve the stud without dissolving any of the alloy surrounding the stud.
Obviously this would be a good solution if the engine was apart.
http://forums.pelicanparts.com/porsche-911-technical-forum/523409-nitric-acid-remove-broken-exhaust-studs.html
Option 2: Locally there are mobile techs in vans that remove broken hardware. That's all they do.
You call them and they turn up with their special tools and methods...
I'm not sure of the cost but recently someone I know used them and they charged him $120 to remove a broken stud on a Nissan engine. It was the one nearest the firewall.
Also... I'm not sure if this would suit your car but our host sells these high quality head studs for a 911.
