777FLY
the only other way in which the fuel system can be open to the air is via the fuel jettison nozzle valves. If these were to somehow be signalled to open, there would be a large diameter pipe, open to air, which is routed via the override/jettison pumps into the same area as the wing tank pumps.
Sorry to disappoint you but the Jettison system does not use the engine fuel feed lines. There is a Jettison pump in each wing tank that pumps fuel into a Refuelling/Jettison manifold. At each end of this is a jettison nozzle.
To Jettison from the centre tank, the normal centre tank pumps must be switched on manually (if not already on), then when jettison is armed two jettison isolation valves open to allow the centre tank pumps to pump centre tank fuel into the jettison manifold. So to get air from an open jettison nozzle into the centre tank the jettison isolation would need to be open as well.