That could be a bit tricky as its probably a taper thread?
Equally, you don't want to be risking bits of metal dust going in the engine.
How overtightened was it? Could you do it with a good quality easy out?
I've used left handed taps for that kind of job in the past with good sucess.
I have used the left hand taps but it aint shifting, so yes tight indeedy, I know what you mean with the metal bits in the engine, thinking of hoover nozzle next to it and dremmel on low speed,.....