Jon...
ok as far as I know, you can have installed in the car but can not be connected up, if cops knows the fact that car carries NO2 BOttle, then they will check it for it. but they dont know what they are looking for? then they dont know nothong. simple as that.
you can get yellow but I dotn think cops cant charge you for it but if they do anything they'll give report to EPA... which then EPA will issue you fine upto $50,000 for the environmental Law as No2 gas would be way above the legal limit in case of accidental release.
over pits? I dont know depends on engineer.
you probably ask danny from here who owns No2 system in his R33