Good question.
But answers will fall into the "what I got away with" rather than "best practices".
Can you start any engine and shut it off before it reaches operating temperature? Sure. But if the oil is not brought up to operating temperature, moisture will accumulate in the crankcase. Over time that will shorten the life of the engine. Same for driving all moisture out of the electrical part of the generator.
So, yes you can do it, but much better if you can combine use of 120 VAC to get closer to that 50% load and minimum of 30 minutes.