Now, the diagram is of the rear suspension, including the brake drum and such, so there are no callouts for rim, tire, tube, or balancing weights--just the brake shoes, return spring, cam, etc.
So I went out to the garage, loosened the stem nut, took the cap off, pressed down--and showered myself with Fix-a-Flat.
After cleaning up the mess, I tried again, and I'll be switched if that f-ing thing didn't move and spring back--a tell-tale sign that there's an inner tube in the thing.
No wonder I've got a rim leak: the inner tube was punctured by the nail, and the Fix-a-Flat couldn't seal the hole in the tube, and the rim was never intended to have a tubeless tire installed in the first place so of course it's not gas-tight.
So now I've got an unholy mess to clean up, because I'm going to yank the wheel off the bike and patch the inner tube, and I'm going to have to clean the Fix-a-Flat out of the inside of the tire.
On the plus side it means I don't need to buy a new tire. This revelation saved me $95!
I may have to buy a new inner tube, though. We'll see how screwed up the old one is--but it's probably just a matter of patching it, and Fix-a-Flat won't hurt an inner tube any. (Same way it won't hurt a tire. It's just messy and sticky.)
I feel a little better. If I can get Og's tire irons relatively soon maybe I can get this fixed! Maybe make it into a nice little birthday project for myself....