Node Access by Menu Position -- Does this exist, or should we build it (and can it be built)?

A client of ours -- a university -- has quite an extensive hierarchical menu structure. They want the ability to take a top-level menu item, such as "Current Students" and control which roles can manage (create/edit/delete, based on their role's permissions under admin/user/permissions) and view the content under that section. Permissions should cascade down to sub-items in the tree unless explicitly overridden. They also need to then restrict access to adding new pages underneath menu items they do not have access to.

Here's a mock-up that describes what we're after, since it's easier than me explaining. :) Also, I should point out that this is for Drupal 6.

I know of Menu Sub-tree Permissions which should be able to handle the limiting of placing things in the menu on node addition, and Menu Per Role which restricts access of viewing. Anyone know of one that does it for create/edit/delete? Is this worth rolling into one of the above modules (once they're ported to D6), or as a separate project, or am I missing a big, blinking cluestick of a module that does this already?

Thanks for any assistance!