I'm envisioning considerable difficulty for the GHTF harmonization folks in arriving at an agreed means of handling this classification if/when the FDA's and MDD's classification systems are merged...due to the considerable differences in social outlook on personal firearms in the US and Europe.
The norm in law enforcement, as far as I know, is for travelers with a legitimate prescription for a given medical device or pharmaceutical to be considered to be excepted from local legal constraints when legally visiting another nation. Prescribed pharmaceuticals for personal use that are not legally available in the US can be brought into the US by individuals on that basis.
I wonder if that will hold true for an individual with a palm-gun prescription, with the gun itself properly declared and traveling in secured fashion in checked luggage or separately in a ground-crew-secured aircraft transport locker, when visiting, for instance, the UK?