All this talk of invisible fueltanks - how about the DA-40 then? This has four tanks in total, two in each wing, joined by tubes so the outboard tank always drains into the inboard tank automatically. The outboards only hold 3USGal each... so anything beyond 6USGal (out of 40USGal) burnt makes the tank look completely empty.
Diamond thought this was stupid too - so they provided a fuel gauge which works like a manometer. Attach it to the fuel drain and you can SEE the fuel in a clear calibrated pipe. Works a dream, albeit a little soggy when you remove the gauge from the drain as the fuel in the pipe has to go somewhere! With a little practice you can make it "on the floor" rather than "up the sleeve"
I'm sure a similar system could be made for the other aircraft mentioned with "invisible" fuel tanks.
MD.