An important issue with electric generation: when wanting to provide power for household items, the voltage MUST be at a very specific phase and level. This is done by the grid by providing a stable current on the high power lines, and in turn is run through transformers and stepped down to the consumer level, the last stop being a transformer typically for each home or perhaps a chunk of homes. These are usually a gray cylinder mounted on a pole, sometimes they are on the ground inside a green or brown or gray box. In any event, steps are taken to insure that the outlets in your home have a nice, clean, steady single phase 120VAC.
In order for a generator to do that, the armature MUST spin at a very specific RPM. And this RPM has to be maintained when load changes. So a portable or semi-portable generator is NOT just an engine bolted to an electric winding inside magnets... it is a
system that makes the above things happen.
Tractors already have an RPM governing system in place, even old ones (although those are more prone to fluctuations when the load changes). But modern electronic controlled diesel tractors have no problem keeping the PTO's RPM dead on at whatever RPM you set them to. So the tractor engine itself takes the duty of much of the genset's needs.
Our TDIs' engines have this ability in the ECU, of course, and the VAG gensets' ECUs do that as well. But the only way to do this with a
car ECU (if a tuner couldn't do it) would be to use the cruise control, which would involve the rest of the car.
So I suppose if you had a giant covered place to put it, you could construct a big roller setup like a drive on dyno, have the generator driven off the rollers, with a fairly heavy flywheel to help cushion load changes, and figure out what the RPM need would be at a given roller speed, and have the car up on the rollers in a higher gear with the cruise set so that the engine RPM was in the meat of the power curve. Which is as a rule of thumb somewhere between peak torque RPM and peak horsepower RPM, although with the TDI it would obviously favor the peak torque.... I'd guess around 2200-2400 RPM. Figure out your roller gearing to spin the generator at probably 1800 RPM. Maybe have a fan rigged up in front of the car to mimic some airflow as seen on the road, probably open the hood, too. The car could run along like that until it needed a PM service theoretically, and you could easily add fuel as it was doing its job... may want to shut it down every few thousand miles to at least check the oil, though.
Still an awful lot of work and still terribly inefficient and overkill. But, a 90hp genset could probably power a small apartment building!