Using a via fence without a top layer ground pour
First off, depending on the plating you use and the size of your microstrip lines, it could be that you want to pour the top metal anyways. For consistent plating, you need to have even, constant density over the PCB. Usually when you have a PCB that has very low copper density on the outer layers, the manufacturer will either ask you to increase density, or they will use a peel-off copper layer that they place before plating and remove after. 

The nature of the microstrip trace is that the current and magnetic fields are usually well-confined around the trace in question. It is hard to say that a via wall is going to help you here. 

Usually you do need copper on the top where the via is, because the electroplating process requires a seed to start going. This is in part why you have an annular ring requirement. 
How far away from your microstrip will these vias be?