As far as I know, combustion isn’t required for NOx formation. If you want to run hotter without producing NOx, you need to eliminate either nitrogen or oxygen. IIRC there are a couple of proposed designs for doing this: combustion in nitrogen-depleted air and chemical looping combustion.
Am I misunderstanding this or are both of those concepts basically "just inject pure oxygen"? And I'm no economist, but running a powerplant on rocket fuel doesn't sound that feasible to me.
Urea injection into the exhaust gas sounds more sensible to me, and even that is seems economically questionable. Fuel prices would need to be pretty high that the efficiency gains make buying urea (instead of just more fuel and running the combustion colder) worth the investment...
Now what? It doesn't burn, since there is no oxidizer. The only way is to inject some. And since you don't want nitrogen, you can't inject air. So you inject pure oxygen, or something like peroxide?
Sounds expensive.