FWIW, I had already done all the updates, crons, repairs, etc. Toggling journaling had no effect.
I have for the past several hours, however, been running without the update process! I ran "sudo killall update" some time earlier this evening and my PowerBook has been gloriously quiet ever since. Hallelujah!
Now, I just need to be sure I don't end up hosing the filesystem by accident.
Everything of dire importance has been backed up. Tomorrow I'm going to reboot and run fsck to see if there has been any damage.
I still have not gotten around to querying Apple directly about this. I'm going to collect some more data over the next few days before doing so. Hopefully if I present it all nicely, I'll actually get a response.