stepen low's recommendation is very sound. Use a specific
oil made for tapping, and apply heat carefully around the outside of the hole to make the hole expand and release the grip on the tap while sliding the tongs of the needle nose into the flutes of the tap. EDM (Electical Discharge Machining) Leeroys suggestion will work too, but is a little more involved; or (?) could the immediate surrounding area the hole is in be cut out and a "Patch" carefully welded in???