From my Operating manual (revision date 1 MAR 03):
GPS receivers utilizing external wires (i.e. suction cups) may be operated while the boarding door is open. At times, during extended taxi delay, the Captain may authorize this type of GPS system to be operated. [In this event someone asks if PEDs can be used, the F/As ask us, and we make a PA authorizing their use until we get near the runway].
GPS receivers that do not utilize external wires may be operated at all times.
The cabin crew is responsible for explaining and ensuring compliance with the policies/regulations concerning the use of PEDs. Their guidance may come from government regulations or company policy. If it is thought the F/A is incorrect, most companies have a list of approved devices in their inflight magazines. That should clear up any misunderstandings. Perhaps there is also a web site that lists these approved PEDs.
I would be happy to see such a passenger in the cockpit early in the boarding process. Then I could brief the lead F/A that you will be using your approved GPS. We could also review our proposed route of flight/ North Atlantic Track together.