Partly on the recommendation from a friend, and partly because I already knew what I had done was pretty drastic, I’ve decided to re-open commenting by unregistered users.
Instead, I’ve installed a new-fangled WordPress plugin called Spam Karma 2. It’s an extremely complex, but hopefully effective, spam management solution. I’ve already seen it eat several dozen spams, so I know it works. One of the things that sold me is that the author claims the program throws virtually no false-positives. We’ll see about that. Let me know if your comment gets eaten.
Self-registration is still enabled if you’re interested, and I’d recommend it since apparently Spam Karma goes easier on registered users.