Allow an author to edit a specific post

Hi all,

I’m trying to figure out how to allow an author to edit posts with specific tags only.

Currently:
I have a post type called “Articles”.
Author can only create articles and edit their own articles.
Only admin can assign tags to Articles.

If an admin creates an article tagged “public” how can I allow my authors to access this article and edit it?

So I’m looking for:
Author can create and edit their own articles + any articles tagged as “public” by admin.

I’ve seen it done in Role Scoper plugin, where specific users are granted access to specific/single posts. I’d rather not install the whole Role Scoper plugin, but am just looking for that small bit of the puzzle! Any help much appreciated!