alt131 - 2:21 pm on Jun 9, 2011 (gmt 0)
Unfortunately you are right - background-images repeat in full, not parts or "slices". I've looked at this several times since you posted the question, but nothing has occurred to me yet. That includes thinking about some of the options available in the css3 "Backgrounds and Borders" module. There has been some discussion about adding a "background-image-slice" property that would do what you want, but no developments on that yet.
So not an ideal response - but a fascinating challenge. Can anyone else think of another approach?