Can a post be hidden from home page, archive view, category lists etc. and viewable only if you have a direct link to it? The blog doesn't have registered readers and is open to public so that would be a mean of hiding some posts from public view without using the password protection.
I found the simplest way - just use Simply Exclude Wordpress plugin. It has the option to exclude each post (or tag, for that matter) from front page, archive, search or feed. It works flawlessly. You can still view the posts by using direct links.