I can't post specific stuff due to NDA.
Lets say you load Scroll View (Panel) scene.
And replace the foreground target object on the scroll bar, with a different sprite from the NGUI sprites. Lets say a smiley. By default it will stretch to the length of 720 which will make it look weird, but it will still run the length of the scroll bar.
However any try to scale it down to proper proportions, will also scale down the length that it runs on the scroll bar.
So how can you possibly accommodate different size sprites and keep the same range?