Synthesizing nitrogen is very expensive (in energy and in monetary price). If this exhaust idea worked you can be sure farmers would snatch it up. Unfortunately it is snake oil. AFAIK, there is no serious study showing any effect.
Using legumes to fixate nitrogen is something that *does* work and farmers are happy to do so if there is a market for the crop (we grow yellow peas as much as reasonably possible). Because organic farmers can get a premium for their other crops, they sometimes grow legumes purely for the residual nitrogen and plow them down instead of harvesting. Unfortunately organic farms requires quite a bit more fossil fuel than modern conventional farming (something most shoppers are probably unaware of).