I've found this and, after some quick tests it seems to clean XML from control characters like a charm (although more testing is needed).
http://confluence.atlassian.com/x/Ly8
(just follow the 3 points at the bottom of the page with your moodle.xml file)
Hope this helps, ciao
P.S.: Some reference to it in docs.moodle.org http://docs.moodle.org/en/Backup_FAQ perhaps would be nice if it works properly.