After the < 5mg/L reading I got from an earlier test, I ran the reference test, which ran correctly. Not that that's much more than a sanity check.
I wondered if I might have the wrong thing in the bag.
A first-level Agromin support person gave me values of nitrate as 2-8.6 milligrams per kilo dry weight of EcoScraps compost mix. That's less than 1/1000 of what I expected! I asked for someone else to call me back. No call so far today.
Further data from EcoScraps directly shows 0.001% dry weight as nitrates. So a kilogram of dry compost (is there such a thing?) would still contain on the order of 10mg of nitrate... right along with the Agromin folks and my tests.
It would appear that this compost is not suitable for LaConte style recoveries.
I now have the name of someone else to contact who may have more comprehensive information on the subject of compost nitrate composition. I will call him tomorrow.
Compost experiments 2 and 3 halted for now.