Toon shader and alphas

Hey guys, so I know this has been brought up before, but with the toon shader is it possible to work in opacity detection through the alpha in an image? I realise that the outline will follow the mesh rather than the texture, but this is more for semi-opaque surfaces such as glass or a tree you want to see through, how easy would it be for me to program in alphas into the existed lighted outline shader? Is it even possible?