Tested this yesterday. Worked well though looks and feels silly to burn gas charging the car.
Here is how it worked. Rolled out my Generac XG8000E generator. It has a 240v 30a connector. I had an adapter from that to a 10-30 (3 prong old style dryer) outlet. Started the generator for the monthly or so test, warmed it up, and plugged in the Jesla. Got green lights, connected to the car, and it started charging.
It pinned the power gauge on the generator, that should have drawn about 24 Amps @ 240volts. Let it run nearly an hour which pulled the charge up from about 80% to the low 90's.
I have to run the generator periodically anyway, so rather than connect the 2 space heaters I usually use as the dummy load I'll use the car now.
Since the 10-30 old style dryer connector doesn't have a ground, the Jesla passed its self test and it all worked. It is possible in other situations you'd have to ground the generator before it would work.