My experiment has failed unfortunately since Reaper uses the center of the vol thumb image to define where the fader is against the fader background, rather than the part you define between the pink lines
I was hoping to make a fader where the actual fader was just a small vertical strip of a solid color, and there was a big trailing block behind it of the same color, then use the z-order stuff to place some clever overlay from another element on top of it when you get past the end of the fader background so it would look like you were "filling up" the fader lane, so the left side was whatever this solid color is and the right side would be the regular fader background....
Worth a try I suppose, oh well.