Precision opto-isolator voltmeter circuit
This wound up being a wiring error. On the board, pins 5 and 6 were swapped. I tried to unswap them, but the circuit still didn't work. I made another board with the error corrected and it worked. My best guess is that while pins 5 and 6 were swapped the output phototransistor on that board got blown somehow. Without the error, the next attempt worked just fine.