Hi,
If you look at the snow tree discussion, you will see that the trick is to insert your surface layer between the default shader for the leaf and the multi shader for the tree.
Go to node view, choose the object, right click on it and go to the internal network. Right click on an empty space and create the shader you want. Then connect it as shown on the screenshot below. This doesn't change the opacity as you can see from the example images. It is not so pretty from close - it would look better if you altered the colors of the image in an image editor. But for an overview it does add variety.
Helen