Well, that was easy. When I pulled the manifold back off, I found that the broken bolt still had about 2 threads sticking up above the head, and was not tight at all. So I was able to remove it with nothing but my fingers. Close inspection revealed slight but distinct necking of the bolt, on the thread right below where it broke. There's no way I put enough torque on it to create any necking, so it had to have been overtorqued previously. That's a little scary. I took a look at the other bolts, and they looked OK, though I didn't reuse any of them.
Hmm. I forgot to publish this post. Oh well, here it is.