First, huge thanks to Jean-François and Marie-Ève at Hanna Instruments for their support! I observed that resetting the electronics had an effect on ORP readings:
In the above image, ORP is hovering around 720 mV before resetting the electronics at around 12:05. I was surprised to see the amount of time it took for the readings to recover (many hours as can be seen here). I didn't observe this phenomenon when using a concentrated solution, so I inquired with Hanna if this might be normal. They responded by sending someone over with one of their ORP meters, and the same observation was made with their equipment. My conclusion is that it has something to do with alkalinity or ion concentration of the measured solution (?).
I chose Hanna's HI 2001 ORP and HI 1001 PH probes for the project. The probes are compact and rugged, and have 3/4" NPT that can be used with readily available PVC pool fittings. They have so far worked very well.
I also got 470 mV solution to calibrate the ORP probe, along with PH 7 and PH 10 solutions to calibrate the PH probe. Calibration with the system is pretty easy: just put the probe in the appropriate solution, and press the corresponding button:
I don't currently do temperature compensation for PH, but that may be something I do in the future. Given that I try to maintain PH in the 7.2 - 7.6 range, the effects of temperature variations are limited anyway.