Sounds like a fun project. Does anyone know whether they made a TDI cabrio in Europe?
When I was researching my A4 TDI swap (I was then considering a 4-cyl) and I called the EPA, they seemed to acknowledge the fact that VAG actually made TDI cars in my model year even though they came in as Passats and not A4's.
The regional administrator I talked to said that the swap would be fine so long as I retained all the factory emissions equipment that came with the car. That said, it's important to note that in my county, they recently did away with emissions requirements, so I shouldn't have any problems there. For registration purposes, I don't have to change my title.
Previously in Ohio, they'd run your VIN at the emissions testing place, then proceed with the test from there. On a quattro equipped car, they don't put it on the rollers, they simply plug in the OBD plug then run the car at different rpm's...
If your testing in DC is the same, they'd pull up your car and want to plug in an OBD test plug and possibly use a test probe in the exhaust as for a 2.0 liter gas motor. The diesel opacity tests are different, as you know. It would appear that you have to somehow convince them that your VIN doesnt' correspond to the engine.
Each county / state varies as far as what they allow and how they deal with "modified" cars. I'd suggest trying to get ahold of someone in your local DMV / EPA offices to see how they deal with situations like this. As with any gov't agency, you might have to make 5 calls before you find out who to deal with on this...
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