You are correct in that static electricity is an issue for any aircraft when refueling. It generally occurs between the tank and fuel nozzle....
A drop tank is part of the aircraft's static charge and a mechanism must be introduced to discharge it.
A lineman sticking a fuel nozzle to an ungrounded aircraft will cause a static discharge.
The question becomes what is going to cause the static discharge in our drop tank?
http://www.experimentalaircraft.info...efueling-1.php