Surely a cheap & simple system would be to continually circulate the fuel from the centre tank to the wing tanks. As long as the fuel is maintained below the flash point (minimum 38C for Jet A-1) flammable vapours should not be generated.
Heavier distillates will invariably have a heavier density, and thus will not "float on top". Additionally, there would be problems with CFPP, freeze point, & MSEP of these materials.