You know, I pulled that plug today and it wasn't as snug as I thought.  I yanked it out pretty easy, and there was no seal on it anywhere, so I bet that thing was leaking (in addition to maybe the head seal).  It seems like a pretty flimsy piece.

Another question.  LWRider over on the Motorcyle Forum (he posts here too, I've seen) suggested replacing the shaft end seal too.  Would that be a good thing to do at the same time?  A thin strip of the rubber had come off of it and was stuck on the head cover, which I had to scrap off.  That left the shaft end seal a bit rough.  I would think that the silicone might be able to fill in the gaps, but I suppose it's safer to replace it, no?