Drilling the bolt :-P
Allways awkward, you�re likely to slide a little bit  = new threads
OK, there is helicoil but.....
I'm not sure weather there is room for it or if it�s possible but I have saved some thread in stupid places ( what is a good place for a bolt to snap ? well maybe if it goes through so that you�re able to reach it from behind and remove it that way :) ) by welding a one size smaller nut onto the broken bolt !
You hold it close up to the bolt and weld in the hole in the nut .
Many times the heat from the welding helps to "loosen" the bolt and it�s fairly easy to remove it with a ratchet...
Worth trying if there is 
1 room enough
2 someone willing to weld 
The important thing is to take a smaller dimension nut so that you don�t weld it to the surrounding material but just the broken off bolt

Good luck 

Calle
-88 Vanagon
Mullhyttan, Sweden