Agreed. They'd have to climb under the truck and know what they were looking for and I've never seen them go further than under the hood and maybe the gas cap. As an update, I put around 150 miles on the truck (mostly surface streets) since the non-foulers were placed and did another 130 freeway miles today just to be sure the OBD2 was ready for testing. It was a good excuse to go check out Hungry Valley and exercise the 4x4 capabilities.. No CEL. I went straight to a smog check in LA and passed with flying colors. The tech commented that the numbers looked really good and congratulated me on the shape of the truck. Aw shucks... Thanks again plh for the non-fouler trick.
To finally put to rest the issue that sparked this thread almost 5 years ago...the DTC P0420 was in fact related, in my case, to a failed precat on the right side. Unable to find any OEM precats on the market, this past weekend I had it replaced with a Miller Cat part #74004 which I purchased on ebay for $150. This particular part has an EO# making it legal for installation on Gen 2.5s in California. I had the new precat installed at Mike's Performance Exhaust and Radiators in LA for another $150. I had an excellent experience at Mike's and would highly recommend his shop to anyone. Then I cleared the check engine light, drove around to ready the monitors, took the truck to get smog tested and passed!