No luck.
I'm feeling like this can't be done without modifying /app/lib/AppNavigation.php
The only place I see the hook hookRenderMenuBar() getting called is in getHTMLMenuBar() which only hooks after getting the top menu (with getNaveInfo(0) )
I'm leaning towards putting an issue in github:
Modify getHTMLSideNav() to add a new hook called hookRenderMenuSideBar() right after:
$va_nav_info = $this->getNavInfo(2); // get third-level navigation (zero-indexed); first two levels are in top-level nav bar
// fire hook
$o_app_plugin_manager = new ApplicationPluginManager();
if ($va_revised_nav_info = $o_app_plugin_manager->hookRenderMenuSideBar($va_nav_info)) {
$va_nav_info = $va_revised_nav_info;
}