To protect the fuel filler neck from dirt, dust and debris, Easy Fuel relies on a flexible rubber seal in the body housing. The system also comes with a handy plastic funnel, which is stored with a vehicle’s tire changing kit, in case someone runs out of fuel and needs to add a gallon or two from a portable container.
from [broken link removed]
i assume that this funnel comes as standard. If so then maybe customs can use that to gain access.
Also, as the technology becomes more widespread customs will surely develop their own methods so that they can check every fuel tank.
edit: forgot to mention, they also can check the fuel through the fuel lines, rather than directly from the tank. I think this is usually done when they suspect a hidden (second) fuel tank.