Thanks for your help and suggestions; after a couple of hours of false starts and attempts at making custom tools, I finally managed it with the help of this grappler tool
(I recently bought it at Home Depot for something else). I attached the hooks to the sensor side of the plug; reached up with my left hand to press down the tab and jiggled the tool with my other hand. When I let go of the tab with my left hand, I didn't feel/hear the usual click so I was reasonably confident that the plug had moved enough so the latch didn't bite; I removed the grappler and I was easily able to remove the plug with my left hand. I was on a crawler with my head towards the front of the vehicle, which was on ramps with axle stands "just in case".
I put this much detail here so it might help someone else when searching for "rx300 how to disconnect rear af sensor connector"
Thanks; I'll bear this in mind for the future. I ended up removing the battery anyway so I could get a clear shot to the front (Bank 2) sensor connector. I wanted a good understanding of the connector design before messing around in back.
The heater on the bank1 sensor is, indeed, open circuit; my 03 has 77k miles on it; seems a similar mileage to what many have experienced.
Next challenge is hoping the sensor is not seized; I put some WD40 on it and will probably let it set until the new one arrives.
Thanks for all the help here; I figure I saved myself a few hundred bucks (so long as I don't crack the manifold).