I like the example. I did something similar with my UI to apply an inner glow from within Skinner's frame factory after realizing that SetBackdrop ist nothing more than a convenience function for 9 :CreateTextures, some :SetPoints and affine transformations via SetTexCoords (rotating the border tiles). You can reverse engineer it pretty easily when looking at the regions of a new frame after setting the backdrop.
skinner.lua
imaging.lua
And I have to agree with Goldpaw that creating clean borders is a somewhat difficult endeavor from time to time. I sometimes had issues with spacing between the border and the background when there shouldn't be any.