Ok, I cheated. I had left the dough to rise overnight, but when I got up this morning it had not so much risen as spread. The only reason I could think of was that the wee beasties that were responsible for the tang had somehow overpowered any yeast that had made it into the mixture. Since the dough was pretty flavorful and because I wasn't terribly interested in just giving up on the loaf, I kneaded in a culture of commercial yeast and let it work over the course of the day.
So anyway, this is the loaf. I've yet to cut into it, but it does smell wonderful.