Outline shader for transparent object

I want to render an outline of an object. There are already some outline shader available for unity

http://answers.unity3d.com/questions/141229/making-a-silhouette-outline-shader.html

This method creates a new, bigger mesh behind the original one in a pass.

However, I want to apply the outline to a transparent object. This means that even the part which should be transparent is altered:

From what I read, I think that this can be achieved doing some sort of multiple pass, but I have no idea how to do this. I didn't get if it is possible to pass single pixel between pass, without render them to the screen, in order to use them for testing etc.