I don't see any problem with using anything, BoB or not
was the vehicle in the theatre at that time?
yes - use it, who can prove one wasn't captured
no - use it, who can prove noone bought/brought one from abroad
the vehicle wasn't even constructed at that time? - use it, a spy stole the blueprints of the prototype and it was built for experiment
of course this rule has to be used with measure, as a king tiger in BoB wouldn't work
one is usually surprised what sorts of stuff showed up in the wierdest places, you never know
or did anyone know that the US experimented with steam tanks in WWI?
or that steam airplanes existed?
(this was not rhetorical)