As many Japanese car owners will have noted, there is a relatively cheap and simple way to eliminate atmospheric pollution.
Automotive Brake Fluid is extremely hygroscopic, a feature which dramatically lowers it's boiling -point and helps the system rust from the inside.
The wiley orientals put an inverted soft rubber "top-hat" under the filler-cap....as the fluid level drops, so does the membrane (retaining -cap is vented, as normal.
Now, Concorde's tanks were lined with a flexible-rubber in it's latter days , so it's not an insurmountable problem materials-wise.
I appreciate there are srructural members, baffles and the like to contend with which may necessitate separate "compartment" bags which could be pinked by a standardised coupling. A metal outer-tank would still be required for mechanical strength and containment of the "bladders".....as fuel is removed, the bladder collapses...any water condensatewould collect on the OUTSIDE of the fuel-bladder and inside the structural retaining tank which would need draining, of course.
This doesn't come without a weight penalty, but IMHO there's a major safety benefit (as anyone who'se experienced brake-failure will testify
)
Going back to the fuel spill-return /heat-exchanger idea,- coaxial pipework with the warmed return on the outside would also insulate the supply from ambient temperature. any failure of the inner-pipe would merely "short-circuit" the rerturn /flow path A leak in the return would be no different to what would obtain in the present layout. again, a safety benefit at minimal penalty.
Appreciated that I may well be talking poppycock,but just sometimes, the bystander sees something the participants miss.