I also found that the left side bearing race on the crank axle is badly pitted, although the bearings themselves and the adjustable cone seem to be fine. This isn't terrible news, since the left side cotter pin notch on the axle was also terribly worn and mangled, and I have an eBay-find replacement axle coming in the mail that should work (watch, now that I've said that, it won't fit!).
On a side note--it's always good to have the right tool for the job. I'd tried loosening the lock ring on the bottom bracket by tapping with a hammer and screwdriver (carefully), but no go. I was expecting a struggle to get the ring off, but I barely had to put any pressure on the spanner at all before it came loose. Just goes to show that a good tool is never a bad investment (minds out of the gutter, people!).