Knobby,
The problem with that idea is that if the fuel in this "reserve" tank is never used, it will go stale, and by the time you need it, it won't work.
The only aircraft I know of that uses this type of design is the Europa. In this case, the reserve fuel tank is on the right hand side of the aircraft, the main tank on the left and centre. The filler cap is on the right. As new fuel is added, it displaces the old fuel out of the reserve tank and into the main tank, thus ensuring that the fuel in the reserve tank is always fresh. It really needs to be designed in from the start, I can imagine it would be a real pain to add it on afterwards.
FFF
--------------