How to remove broken bolt in Chain Tug?
Unless I'm missing something, you should just be able to remove the chain using a chain tool, and then slide the wheel back in the dropout.  This would free the chaintug from the back of the dropout, then you could rotate it down out of the way and be free to slide the wheel forward and remove it.  Once the wheel's out, take the chaintug off the axle and have the full suite of machine tools available to drill out or otherwise remove and replace the broken bolt at your leisure.