Yeah I don't know if it's really possible to get much from doped jet fuel at the height range commercial airplanes typically fly. Just popping my head into the topic on Wikipedia, it seems atmospheric sulfates do block some amount of sunlight, and jet fuel contains some amount of sulfur. Not sure if the global effect from jet exhaust is even measurable though. You wouldn't need many airplanes in the sky to distribute more potent reflective particles described on that Wikipedia page, but it would be a whole lot more obvious.