FH1100 Pilot,
take this for what it's worth, since I haven't ever flown a 54 or 64, but have much experience flying other heavy type 1 Sikorsky helicopters on fires.
We always hot-refueled at 100% flight idle and NOT at ground idle, since many systems require AC power and the AC generators drop offline below a certain NR. You would have to start up the APU if you wanted to reduce the throttle quadrants to idle and still maintain power to the various systems and cockpit indications.
I've spent much time at a helibase next to a 54/64, but can't seem to remember if they reduce power to ground idle during refueling or keep it at flight idle. Maybe someone with the appropriate experience could enlighten us.