At the moment I have no time to look up this. But the built in navigation has a lot of other drawbacks. e.g. the list grows too long in most cases with lots of things you do not need. And the collapsed/expanded status of the nodes is not preserved over any page and hence not even across user-sessions.
That is why I programmed my own myCourses and siteNavigation blocks in Moodle 1.9 and I just have finished reengineering them for Moodle 2.3
You can find them in these forums. Rosario