Yah, put simply - the customs guys are now very well aware that a defender should not have an airbag steering wheel or a TD5 or Puma motor.
It wouldn't surprise me if they don't have a simple visual test for defenders like they had with the austin / morris / Rover mini coopers where folks were putting old vin tags on '99 rover minis and importing them as mid-60's examples, complete with EFI, AC, airbags, SRS, headlight levelers, etc, etc, etc... Images and videos of these being crushed sure put a halt to the number of "pop-rivet restorations" that came to the USA.
Right now, every defender that comes through is probably compared to a "black book" with pictures of the different motors from the 2.25 gas / diesel and carb'd v8 on the "ok" page; 200 / 300tdi on the "perhaps" page, and the TD5 / Puma motors on the "send-it-back-or-crush-it" page.
Same for the dashboards - pre puma on the "ok" page, and the much nicer Puma dash on the "bad" page.
Last defender I imported was a '83 that was all original - except I had a 200tdi fitted. I had no issues, and didn't consider it a risk at the time. Now, I might think about it - perhaps based on what the rest of the ladnrover physically looked like... Come 2016, the 200 TDi will have been fitted in defenders 25years old, until then, even these common upgrade motors should be considered a "risk".