In Moodle 2.2, this was achieved by setting the class of the anchor tag using the visibility propery of the asignment (/mod/assignment/index.php) :

//Show dimmed if the mod is hidden
$class = $cm->visible ? '' : 'class="dimmed"';

Please look at patch.txt for my proposed solution.

1. Create an assignment
2. Under Assignment settings, set visibility to Hide
3. Make sure the assingment is hidden ( dimmed) on the course index page for Teachers and compleltely invisible for student
4. Click on the Assignments link from the Activities Block from the course page
5. Assignments are invisible for students as expected but are visible ( not dimmed) for teachers

In Moodle 2.2, this was dimmed for teachers as we would have expected.