Can't see from the photo, but presumably if this is a tap it has some longitudinal grooves in it for clearing swarf. Occurs to me that if you can locate the broken ends of them you may be able to improvise a tool with which to apply leverage .. a small, tough, two-short-pronged tool? Re-shaped screwdriver bit or something? If it's a tapered tap, you may have a better chance than if it's a straight one. A long-shot, and a labour-intensive one, but fairly risk-free...if it don't work then it don't work. Just an idea..I'm not an engineer and have no idea what 'spark erosion' is.. take it for what it's worth. Good luck!