I found difficult to work with Blender for me Zbrush is a lot easier, but by reading your explanation I got the feeling that modelling, retopology and the conceptuals which seem crucial to create mesh clothing cannot be done only using Zbrush.
Note that I have just started using Zbrush and I have no idea what I am talking about.
But for what I could understand in order to create mesh clothing for Second Life you have to use blender or have to learn how to use the other programs to complement Zbrush right?