I used an exacto knife to cut across the rail, and a chisel to pare back to the cut, repeatedly, until I was paring the last paper-thin shavings of wood off the dookieschmutz below. In this process I slipped at one point, and gouged the fiberglass. I will have a picture later, if it came out.
The gouge had to be repaired, but repairing fiberglass is pretty straightforward. You sand it all off to the bare wood, cut a fiberglass patch that overlaps the remaining fiberglass by an inch all the way around, epoxy it down, sand the edges so they aren't bumpy anymore, put a second coat of epoxy on, and then to the varnish when the epoxy is cured. Happily, it turns out that I don't have to wait for a full chemical cure with this epoxy-varnish combination; 24 hours seems to be good enough.
I also had to sand the dookieschmutz off the hull, which went better than I had dared hope. I got pretty much all of it off. The paring and sanding took about five hours, which was less than I'd feared. Varnishing the sanded bits was straightforward enough, and I coated the seat rail extensions and the back thwart first with epoxy and then with two coats of varnish.
This morning I drilled the holes and screwed seat and back thwart in place. I am very happy with how the back thwart came out; I think it has a much more graceful curve than the old one, and I think the way I carved the part that contacts the paddler's back will be much more comfortable than the plain roundover I used on the old one. We shall see tomorrow morning, because Vonda will be coming at 7:30am to help me put the boats on the car so we can go paddling.
Wish me luck :-)