Jason, here is what I found out for PA
There are no laws making it illegal for PA safety inspection or emissions inspection.
There is nothing the PA's vehicle coded that states that having nitrous in your vehicle, armed or disarmed, is illegal.
However, many municipalities have their own vehicle code that can be more stringent, and can make nitrous illegal.
I know in my area I would be good because talked with a few people that I know locally that have n2o in there vehicles and have gotten pulled over for other violations and the cops commented about the n2o but did not give tickets and one cop even said that it is legal to run on the street in PA, as long as you are not speeding or racing.