It's easy to just to rip off the section with the flats, probably leaving your vise grips uncomfortably close to the rim, which I'm sure you'd rather not scratch...
I like the spoke driver idea, and where that fails, you can mostly drill out the head, maybe finishing the job with a punch when you're >90% through. You can probably crack off any remainders that won't unscrew by smacking them with a hammer on an anvil. A bit of oily scourer action to clean up the threads, chase with a lubed nipple.