I borrowed a computer and a 4 channel picoscope from work so I could check out the oxygen sensors on my Camry. I back probed both of the sensors and started the car and let it come to temperature. The waveform for the upstream sensor was normal. Voltage was normal and it cycled up and down the way it should. Meanwhile the rear O2 sensor gave a constant voltage, not even a slight variation.
So I did a little test by feeding some propane into the intake and keeping an eye on the laptop. The upstream jumped and immediately read rich. Nothing from the downstream- bad sensor. That might explain why it took so long for the monitor to reset after I changed the catalytic converter last year, the rear sensor was probably getting lazy and now it's totally dead.
So the sensor is definitely bad, and it looks like RockAuto sells a Denso sensor (OE is Denso) for around $60. I'm gonna get one of those ordered and I'll go from there.