Hi David,
This function seems to center the chord names (root name and suffix) precisely on the leftmost edge of the notehead. While I feel that this is preferable to LilyPond's default setting, I wonder if there's a way to nudge the chord symbols about 1/2 a space to the right (I'm sure there is, but my total ignorance of Scheme and music functions prohibits me from figuring it out).
What you really want is a Scheme function to actually centre it on the notehead, regardless of width.
A similar thing was being done with lyrics -- perhaps this thread (and the authors thereof) can help:
<http://lists.gnu.org/archive/html/lilypond-user/2010-02/msg00573.html>
Cheers,
Kieren.