You might try grinding a flat spot into the flush piece and using a screwdriver, but I doubt that is going to work if a pipe wrench broke off.

I suggest heat.

There's an article around here, or on turbobricks about using a big nut (several of them) and a wire-feed welder to build up material, then weld that material to a nut so you can put a wrench on it. The heat should break a lot of chemical bonds and will deform the threads to make them easier to remove - hopefully.

If you weld on the car, unplug your ECU's!

Uh.... now that I think about it though, that article was about removing bolts from the head - aluminum won't weld to steel, but steel will weld to other iron-based alloys at least somewhat. My parents mechanic was able to weld a bolt onto a stripped head. I think both the pan and bolt were steel and the results were all right. Just don't heat the bolt past cherry red when welding.

Good Luck!