The setup turned out to be quite easy. I loaded the furnace to the brim with charcoal, lit the pile from the bottom, used a wooden wedge to keep the duct in the tuyere, and sat down to watch, periodically removing the vent to reload the charcoal.
At the beginning of the first reload, I began to hear a curious sound, a lot like popcorn popping. Those more knowledgeable readers will be wincing right now, but after checking that there were no cracks or faults developing, I assumed it was some impurity in the charcoal and blithely continued. The next odd symptom was that the ash didn't seem to be burning as completely. There was a large accumulation of unburned crap in the bottom of the forge, and it continued to grow as I fed more charcoal into it.
The third symptom was clear, however: the edges of the tuyere cracked and widened it to about 200% (and later 300%) of its original size, causing the duct to drop out. At this point, with the better view into the forge afforded by the (now quite large) tuyere, I could see that the accumulation of "ash" was actually an accumulation of spalled clay. The popping noises were caused by fire-hardened clay cracking off as less hard (and still wet) clay behind it generated steam and popped it off.
The clay body was still too wet, and I was eroding my forge from the inside out. Below are some pictures of the whole thing, the inside, and a close up of one "edge" of the tuyere
Well, I was ready for my vacation anyway :)