Capt Quentin McHale,
As far as the warm fuel and IDG theory, it doesn't happen, fuel does not return to the tanks from the engines.
Please see
Fuel Recirculation System | IDG Cooling
& Page 115 of
http://www.academia.edu/8535109/Training_Manual_A319_A320_A321
Lufthansa Technical Training table suggest that up to 500Kg/hr can be returned to the outer fuel tank if the IDG oil Temp >93 deg C.
If the fuel pressure was greatest from the tank pump in the RHS, then fuel could be transferred into the LHS outer fuel tank via the IDG cooling flow mechanism as clark y suggests.