Ask, (or maybe in my case, whine) enough and ye shall receive.
Yesterday the nitrate monitor arrived in the mail! "Woot!"
It looks like it's immune to the sulfur interference. I tested the tank and it read 26ppm and I measured the reactor effluent and it read 13ppm.
So maybe this thing has been working all along... I guess the lesson is learned, you need to take a leap of faith (or find a test kit that's immune to sulfur interference, and FWIW that's neither Salifert or Elos).