Thanks for all the responses. I stripped the bike last night, and it turns out the BB is BSC or BSA or ISO or whatever you choose to call it! I should have looked before I asked: Tange 1.37 x 24 is stamped right on the cups.

At any rate, the adjustable cup and lockring came off easily, and the threads are great. No rust on that side. The fixed cup, however, is being stubborn. Water seems to have entered through the drive side spindle hole. The cup and bearings were swimming in a soup of rust. Peering in from the left side of the BB, it doesn't appear the rust is bad, but there may be enough that it's keeping the fixed cup frozen in place. I drenched it in WD-40 several times over the course of a couple hours, but it still won't budge. I tried securing the flats in a vice, but it kept popping out when I applied torque. I finally gave up, but sprayed more WD-40 and another penetrating oil in all the cracks. I'll give it a go again today.

As far as preserving the paint - I think that would be a tall order. I'd say a good 5-10% of the frame has surface rust, and most of the decals are gone. Some of the most visually appealing areas, such as the AD stamp on the seatstay pictured above, would need to be sanded and cleaned. I can clean and touch it up, but it won't look very good. But as others have said, it will make a decent rider.

Actually, now that it's stripped, I'm having sacreligious thoughts. I do like the looks of a stripped frame, and this bike has absolutely no brazeons...a fixed gear nut's dream!