Well, the good news is that the broken threaded part came off in one piece. The bad news is that it dropped into the cylinder. I spent a pretty good amount of time last night with an inspection camera rigged up with a hook to grab it, and have no problem getting hold of it. Unfortunately it's too large in it's current shape to pull back through the hole, so I'm going to attempt to reshape (read; smash with a wooden dowel) the piece inside the cylinder. If I can't get it done in 2-3 hours, I'll just pull the head. I certainly don't want to if not absolutely necessary, but it would give me a chance to clean up the engine and refresh all the gaskets.
I'll post pics/video from the inspection camera soon.