Here in the U.S. you can get it written into your 135 ops specs to allow the pilot to leave the controls. I had it in Hawaii when I flew there. We had FAA approved procedures for it, and also had procedures for ground crew to do a "control hold" if we needed to run to the "blue room".
It read words to the effect of:
"Exiting the aircraft with engine running. The pilot may leave the pilots seat and remain within the immediate vicinity of the helicopter in order to check for fuel levels, oil levels, seat belts, headsets, etc."
In fact, my op specs required the pilot to "visually inspect the fuel level in the tank", so one was in violation if you did not get out and check.
I still have copies of them somewhere in my office, they are standard for all tour helicopters in Hawaii that conduct engines running re-fueling.