You can look here to get the EPA certification data for vehicles. In 2003, the TDI Jetta wagon 100k mile federal emissions were: CO, 0.4; HC-NM, 0.05; NOX, 0.54; PM, 0.05 (I assume these are all g/mile).
Well, the law simply states that the "clean fuel" needs to be cleaner than a conventional or reformulated gasoline fueled version of the vehicle with repsect to any one or combination of oxides of nitrogen, volatile organic compounds, carbon monoxide. So I'd get the ratings from that same EPA website for the 2.0 gas Jetta of the same model year and use that as a comparison.
Don't expect a decent answer from the Dept. of Environmental quality. I don't think they are the ones that even make the determination. DMV seems to. I've sent a bunch of requests to DMV for all their guidance documents for determining what constitutes a clean fuel vehicles and I've gotten the run around. Under the administrative procedures act of VA they are required to provide these documents so its really a bunch of BS.