Restricting Specific Users to Categories

I am trying to use WP as a CMS, and have been pretty successful so far although there have been a few obstacles along the way.

I am currently investigating the possibility of restricting what ‘sections’ (i.e. categories) a user can post to. My site is setup as having several top-level, mutually exclusive categories, and ideally I would like a user to be responsibile for one of these sections.

I have heard about Owen Winkler’s Limit Categories and Role Manager plugins, however I have been unable to get hold of the Limit Categories plugin to try it out. Also, I think this approach may rely on associating user levels with categories, which is not exactly what I need. I need users, of the same level, be restricted to different sections (top-level categories) of the site.

Anyone got a solution or suggestion to this problem?

Many thanks,
Steve