It seems that with each new warp comes a new problem to be solved. After getting a 33-inch-wide warp (that's over 1500 ends of yarn) threaded, drawn through the reed, and tensioned, I was checking for errors by lifting each shaft, and discovered a major threading error: in one particular 1-1/2-inch-wide block of threads, instead of threading them on shafts 11 through 15, I had threaded them on shafts 10 through 14.
I had to go away from the loom and think for a bit.
Can you say string heddles? Eighteen of them. Here they are. They seem to be functioning all right, and the weaving is now progressing.