A few ideas immediately came to mind:
Fill the "trunk line" will the coolant, instead of just having a coolant line.
Or, easier yet, insulate the trunk line (2-3" PVC?) and run a return pvc line back to the keezer, so that both ends of the PVC loop are inside the keezer. Put a supply and exhaust fan on the ends of the pipe and forget the water and pumps. If you did pick up some heat in the PVC run, you can always lower your keezer temp a few degrees to compensate.