You mean like for 5-15 minutes, or for 2-3 hours?

The risk that you put yourself in doing this is immense. The FAA just needs a preponderance of evidence when you go to apply for a certificate or rating, and if that engine required an overhaul way before TBO or something else bad happened to it, you can bet the owner is going to turn you in quick, no matter how friendly he seemed. Then the questions will start. There's a multitude of ways it could go bad from there. The problem is not sitting there and logging it as you say, it's when you apply for a certificate or rating, except your actions may also be wreckless and you may endanger someone else that uses the airplane, by that accelerated engine wear. Cowlings are meant to move air that is flowing at a certain speed, it's not just about the prop, it's about the air flowing over the gaps/cowl flaps and "sucking" the air through the intakes. While a small 150 engine is likely made with enough cooling to run for a while on the ground, it's still a bad idea. The risk to payoff doesn't seem good IMO.