I was beginning to think that it couldn't be fixed. I ended up buying a new tach drive unit from Jeff@Z1 ($40). Had him throw in a couple of tee shirts, too. He's got a sharp-looking grey one now.
Anyway, the new unit had two o-rings on the body. My stock unit only had one.
The new piece was REAL hard to install. I coated the o-rings with o-ring lube... the same goop I use for the seals on my swimming pool pump. I pushed and twisted the thing, and I had to put the bike in gear because I was pushing it backwards on the side stand. Finally, the second o-ring slipped in, and I screwed it down and reconnected the tach cable.
I put 140 miles on the bike last weekend, and not a drop of oil on the front of the engine.