the most logical means, outside of using a different kind of extractor maybe...... My only suggestion I can think of would be to use a left hand threaded tap and basically tap the inside of the hole until the tap bottoms out and use it to try to turn the bolt loose. With the size of the hole in those bolts the tap should have a stiff enough shank to possibly break them loose.
"If everything seems under control, you're just not going fast enough." - Mario Andretti