I used NGUI free version (2.7) with Unity 4.3. I have tried to use clipping to restrict the area of viewer by using clipping to hide some content of Unity Sprite. However, it did not work. If it's NGUI own sprite, it worked. Is there a way to use clipping with Unity Sprite? I have done some asset with animation with unity sprite (2D) but it cannot be clipped.
As far as I know, Unity sprites supports became available with new NGUI versions (3.x). Previous NGUI was developed before
Unity 4.3 with 2D features was available, and old NGUI is not under development for a quite long time.
So, you can either move to new NGUI, or go into shaders code and make use of
UIPanel alpha values. Maybe you could have a luck and by setting some NGUI shaders for Unity2D sprites you will achieve the same without spending any time on improving your shaders? That should be a good start.