if that indeed works as claimed, wouldn't that also sequester CO2 from the exhaust?

My goodness. That's a lot more technical knowledge on the subject than I could hope for. But reading your post does give me a more distinct idea of the problem. I wonder if saturating the seed with NOx is what's causing the fixing/fertilizing to take place? Do you think this is a technique that could be used in the US or other places around the world with certain crops and farm sizes?

let me quote John Holbrook - one of the experts on the issue:

There is no question that ammonia can be produced on a large scale using
renewable energy. The best example was Norway, where the company Norsk
Hydro used hydroelectric power to produce NH3 for six decades in the 20th
century. At peak capacity, a single Hydro plant was operating 150 MW of
electrolzyer capacity, which generated 64 tons of H2 per day (equivalent to
64,000 gallons of gasoline) and 365 tons of NH3 per day.

The question is not really whether large amounts of NH3 can be made using
wind, solar, or hydroelectric power, it's whether it can be done cost
competitively. The Norsk Hydro plant was mothballed about twenty years ago
because the electrolyzer approach for producing NH3 just could not compete
with NH3 produced using cheap natural gas. The relatively low efficiency of
the electrolyzes and their high capital costs essentially put them out of
business. It simply is cheaper to get your hydrogen for ammonia by
reforming natural gas than by cracking water using electrolysis, or at least
it was at that time..

The landscape has changed a bit for electrolyzer technology since then with
new technologies such as PEM electrolyzers, and new companies such as ITM
Power (UK) and GE getting into the area. In both of those instances, the
market driver has been to supply H2 to the Hydrogen Economy, not to
manufacture NH3, but the technology improvements and lowered capital costs
would still apply to NH3 production.

gee, i have to admit that i must have been left behind since i didn't attend one of these k12 schools. all i know is that the dominant reaction in the combustion is:

4 NH3 + 3 O2 -> 2 N2 + 6 H2O

Has anyone actually used ammonia in any kind of engine in an extended application?

people used ammonia in trucks and buses in Europe during WWII and never heard of any report of engine corrosion. GM and UC berkeley conducted extensive ammonia ICE researches during 1960s and 70s for the army, reported specifically that no corrosion was observed in the engines. there is a pick-up truck fueled by ammonia in operation for years now in Ann Arbor, Michigan.