I submitted a module to support backward iteration in Unicode strings in November 2010, and then a revised version based on Bruno's feedback in January 2011. After I received no feedback at all on that version, I submitted the same thing again in February 2011, which also gathered no feedback at all.
Here is the repost from Feb. 2011: http://comments.gmane.org/gmane.comp.gnu.libunistring.bugs/69 I'd still like to get this into libunistring. Please, if the current maintainer is interested, let me know and I will rebase and repost again. Thanks, Ben.
