Sound like an interesting, but I think it would be very difficult, because content is more than just posts:
- Post authors for the demo content must match existing users.
- Taxonomies (tags, categories, nav menus) are bound to existing content.
- Comments need proper associations with post IDs and post types.
One way to do what you want:
- Create copies of the existing tables (posts, post_meta, comments, taxonomy tables) with a different prefix.
query and replace the table names when you want to show the demo content.
- During import, map existing users to the new content.
There are probably a billion ways how this can break. Multi-site and unusual database setups are obvious. :)