Nick, I've heard of this happening to other MDXs. I can only guess that it's because a plug wasn't tightened properly (and that can happen to anyone). I really hope you'll be able to get a new plug in there without TOO much effort. IF you're really lucky, the new plug - or tap - will clean up the threads without depositing too much debris into the cylinder. I suppose there HAS to be a mechanic's trick for extracting small amounts of shavings - maybe a piece of yard coated with something sticky fed in through the spark plug hole?
Before using a tap, etc.:
Set the bad cylinder to TDC on the compression stroke. Both valves will be closed, piston at top.
When you are through, blow compressed air down the plug hole through 1/4" I.D. rubber or plastic tubing. Protect your eyes. repeat until nothing more comes out of the hole.