Stephen, you got it. The only thing I was thinking is if the slices require full x,y coordinates or only margins, similar to the current box model.
The simplest way to implement this is just by using margins because the “center square” is always in a rectangular shape, so:
background-slice: 3px 2px 1px 4px;
would actually make sense.
If we use x,y coordinates it opens the floodgates to different shapes, which while could bring very interesting effects might be too much to ask for the first draft of this.
Stretching rectangle/squares is pretty straightforward, but stretching irregular shapes might make some people a bit queasy =P