Limit contributor role to editing certain pages (& their ancestors)

I have created simple pages with children (not custom posts) and I would like to find a way to limit a single user to only be able to edit a particular “section” (the parent and its children).

Role Scoper plugin is far, far too complex for what I need to do; I’ve literally spent HOURS on it and can’t get it to do a single thing.

I have used a few other plugins but they just give access to all the DEFAULT permissions, don’t allow you to create new permissions – like limiting page editing to a page id/tree etc.

Can someone show or point me to a custom solution for this? I am fine with putting some custom code in the functions.php or whatever. I already have the roles created; just need assign the certain limitation/capabilities to them. I am fine using this solution in conjuction with a simple plugin (like the Capabilities plugin, which is nice for adding/disabling base permissions on the default roles).

Can anybody help, or point me to a tutorial that explains how this all works?