Just my two pennies but are you not forgetting something very important regarding "hot refuelling" and that is the fact that the rotors turning are increasing risk of static. You can see this clearly when slinging or hoisting at night or low light. The few accidents I have heard of happened due to the static between the nozzle and the helicopter. I haven't heard of any where the engine exhaust actually caused a problem. I would say that proper grounding procedures are vital and very important. We "hot refuel" on a daily basis with no problems at all.