Urethane rubber day. Very smooth, all things considered. Having done it before and having an assistant really make an enormous difference.
One thing that seemed like a brilliant idea, but which didn't work was...
...inserting short lengths of wire into the clay with markings at about 3/8ths of an inch out, so we could keep track of how thick the rubber was. It's a fine idea, but in practice the wires were quickly coated with black rubber, which made them useless. Maybe next time I'll mark them with a bit of masking tape, and see if that works.
Another new development is filling the undercut with foam rubber. Seemed to work great - the rubber sticks to the previous layer, and then is coated by the next. Once I saw how well this appeared to be working, I may have gone a little bit overboard, filling "undercuts" that were hardly worthy of the name. Hopefully it won't all bite me in the ass later.
So that's it. Mother mold tomorrow. Hopefully just as smooth.