Without a 1-way valve, wouldn't it be dumping most of that oil back to the sump (low pressure side / path of least resistance) as it would attempt to equalise pressure either side of the Tee? I suppose that even if the pressure is drastically reduced, as long as there is some oil available from the accusump it would be better than an air pocket in the galleries.

The pump will prevent any oil just draining back into the sump because it rotating and still producing a head pressure (If that's the right term).