So I wrote a WordPress plugin, called Post In Post.
It’s basically a simple plugin to provide admins with a way to browse and insert post content inside other posts.
The main aim was to try to integrate into WordPress as best as possible. It extends the editor interface and provides a browsing tool.
Content can be inserted inline (i.e. copied into the post content) or as a shortcode (i.e. the content will be inserted at rendering time), and either as full length text or as an excerpt.
The plugin can be downloaded here: http://wordpress.org/extend/plugins/post-in-post/
The source code is also available on github.
Please submit issues through github if you run into problems.