Why not use electron beam to treat the cast iron parts, especially for sliding parts. You can use the beam by case hardening (without subsequent machining) or by remelting (required subsequent machining, but excellent wear resistant).
I think your problem may be the over pressure of your furnace (10 ~ 3 atm). The ZrO2 oxygen probe normally works at a pressure of ca. 1 atm. The accuracy of this sensor is quite high, actually. Furthermore, you can use a little air as additing gas for better responsiveness of your sensor.
you want to compare the results to get the difference between serveral nitriding potential. I think you should analyse the compoud layer too. For that you can use Glow Discharge Optical Spectroscopy (GDOS), very fast and quite eccurate to get the N-, C- Profile in the Compound layer, or...