brandnu6 - is there a thread with info on your hood vent? I find it interesting.
anyway - let me think out loud for a minute....if you attach the aux fuel tank to the vent line, it will just spill into the tank - so you would need a solenoid valve, or you would need to use the pump and pump it into the vent line. If you attach it to the return line, and if the aux tank is vented - same thing. But if the aux tank is not vented, it won't spill in until the gas tank is empty since the return is below the fuel pump suction. So you'd still need to use the pump. If you attach it to the fuel feed line, it will need to handle the pressure and if it is vented it will overfill, but it would allow fuel to drain back when off.
So first - is your tank vented?
Assuming it is, I think if I wanted to try this experiment I'd use the pump to pump gas on demand (switch or find a way to make it trigger off level) into the return line.