I drilled out the remainder of the bolt today. I was surprisingly well centered initially, though the final hole that I'm left with is not quite the same as what was there to begin with. Upon further inspection of the bolt, it was not the correct / original bolt, so this may have contributed to it seizing in the threads.
I am going to tap the hole to M7 size. I'll use a heli-coil if tapping is unsuccessful. I'll also be using "The Right Stuff" sealant for extra insurance against leaks.