I can't speak with absolute authority, because I don't work for the DOT, but it is my understanding that the certification process has to do with safety (windshield and side glass, passive and active restraint systems..etc), and emissions (we have really crappy gas here). I don't believe it has anything to do with what side of the car the steering wheel is on.
Case in point: A friend of mine has an 'illegally' imported Silvia and the state of Illinois doesn't say boo to him about his steering wheel, as long as he passes his biannual emissions tests. My uncle's classic Mini was properly imported in the late 70's, but is exempt from emissions testing. Same deal, no issues with the steering wheel being on the right.
Motorex legally imports R34 and older Skylines, and again, no mention of issues with the steering wheel, just with safety and emissions. I tried doing a quick search on the USDOT's web site, but all I got were references to a phone number for more information, and instructions to contact local state DOT for state specific questions.
So no, I do not think it is any harder to import and legalize a RHD car (R34) than it is to import and legalize a LHD car (BMW 1 Series)