I recently came across a job where I needed a filter on the admin menu, and could not find any such feature after extensive searching.
I have managed to come to my own solution, however it would be great to see one in WP.
By editing the _wp_menu_output function in wp-admin/menu-header.php I changed the output to concatenating on a variable, and then was able to apply a filter (I called it admin_menu_output) which I output.
This adds loads more functionality and customization to the admin menu.
In my case, I wished to add submenu seperators, so I created menu items where the HTML came out something like:
Then I filtered this and was then able to achieve my goal.