The books I have say the pressure would be low and erratic not necessarily none.
I know that my x block and the r race blocks have a feature cast in to reverse feed the main oil galley. The idea there was opposing pressures cancel each other to stop velocity in the galley to better feed the main bearings. That could partially be what happens without the plug. As I have said, without the plug there is not always a clearly defined flow path which for lack of a better word could cause erratic flow.