Well, there is more to NOX emissions than an EGR valve. There are design considerations having to due with "dead space" in the chamber, cam design and combustion efficiency that can also affect NOX. This is why newer engine designs have done away with the EGR valve: They are designed to be clean enough not to need it. One way to limit NOX and HC is to use a ring package that is closer to the top of the piston than normal. This reduces the amount of space unaffected by combustion, as does tightening the quench area. High swirl head/piston combinations also work to this end as well as having a cam with a wide LSA. Now, some of these things are the opposite of how people build performance engines as they work against high RPM horsepower and durability under boost or nitrous use. However, they are things to consider if building an engine that has to sniff clean. The wide LSA cam allows more cylinder pressure at low RPM's and also allows combustion to have a longer time interval at low speeds. It also can be used to smooth out idle quality and increase low RPM torque and fuel economy. Likewise, hypereutectic pistons are preferred for emissions applications because they can be run at a tighter wall clearance than forged. This also works to decrease dead space, and lowers the amount of unburnt hydrocarbons and NOX. It is also counter intuitive for ultimate strength, but is something to think about in a smog Nazi state.
I would also caution people to think about the upcoming "Cap and Trade" legislation (if it passes the Senate- (S.2191)) which will limit the amount of Co2 you are allowed to emit. This will be achieved with higher taxes on fuel, and an effective doubling of electric rates. You may also see a CO2 tax on cars like the British have as a way to use taxation to force people to use greener technologies. If this happens, it could curtail your ability to drive a modified car as municipalities will be forced to enact strict emissions tests to bring their areas into compliance.