I don't know if it directly answers your question, but NOx emissions are dependent on combustion temperature I believe, so the ECU could potentially adjust fuel

I don't know if it directly answers your question, but NOx emissions are dependent on combustion temperature I believe, so the ECU could potentially adjust fuel and timings to create conditions that don't spew NOx, but don't perform as well either. Then when on the road, it opens back up to increase performance but liberate more of the NOx that otherwise might be bound up in less harmful substances or caught by the particulate filter/catalytic converter.