Kramer on Seinfeld invented a similar thing for a bladder within the hold of oil tankers so that if the outer metal breaks, the interior is still intact and a spill is avoided.

I am uncertain about the 747's fuel system but I am very familiar with other aircraft fuel systems. The bladder idea is already incorporated into many aircraft designs. The problem is that the sensing systems that determines how much fuel is in the tank is electric. In fact, on the aircraft that I fly (Cessna 340)the sensing system is like a giant capacitor with the fuel being the dielectric. Personally I dont like fuel bladders. They are easily damaged by installation and/or vibration. The materials that it is made of get old and form cracks. And the fire/exsplosion damger associated with fuel vapor is in my never to be humble opinion hugely over rated. I am not convinced that fuel vapors brought down TWA 800, but assuming that it was, I think that filling the tanks with inert gas is the cheapest and safest way to fix the problem.