After struggling for a while with adding custom links to the admin bar for my custom theme-options pages I finally found the error.
Apparently for existing parents, such as Appearance, Add new and Updates, it's critical to declare a priority for the hook.
This won't work:
This will work:
add_action('admin_bar_menu', 'modify_admin_bar', 99);
Notice the third function argument. It doesn't make a difference which number you choose as long as it is defined.
In the future I'm hoping the admin bar automatically adds custom sub-pages like the admin sidebar navigation does.
Anyway, good to know.