After experiencing the safety check plan that is used in Missouri, I can say I much prefer it over the smog check. Though part of that inspection involves making sure all EPA mandated equipment is on there, and a Diesel swap won't pass either.
The problem with California's setup is that even if I wanted to modify my cars to be cleaner to the environment by doing anything other than changing my air filter or running a bit less oil in the sump to keep windage down, I'm screwed. Water injection to keep NOx down? Illegal! Converting to an alternative fuel (regardless of how good it might REALLY be) is illegal unless you can find a kit that's been certified, and you HAVE to be a company intending on selling the parts to be allowed to get certified.
Essentially California makes it illegal to be a shade tree mechanic with a slant toward making your vehicle a bit better to the environment... or worse