Show custom data instead of 404

I’m currently developing a plugin to display some custom data on my wordpress site.

For example I have a data named abc. If a visitor try to access localhost/wordpress/abc, it should display the data programatically from code. But if a valid post url is accessed like localhost/wordpress/existing-post it should still display the post.

I’ve currently looking into trapping 404 redirect using template_redirect action and redirect_canonical filter, but the solution doesn’t feel right. Also I’m looking into using custom post type but not sure whether it’s suitable.

I’ve tried by create a post for each data, and use the_content filter to display the detail of the data, but this method generate too many posts so I’m looking for a cleaner alternative.

The data I have is generated in the background over time. So it will grow to thousands or tens of thousands in the future.

Thank you.

EDIT:

I’ve adopted 404 Tools plugin by toscho. These is the essential part that I’ve modified and use

add_filter( '404_template', 'myplugin_display_data' );
public function myplugin_display_data( $template )
{
    status_header(200);
    new self;
    //For now forward everything to my own page for testing purpose
    return dirname(__FILE__) . '/myplugin-page.php';
}

Now I notice that the output will have error404 in part of the CSS class of the body, which still render the output like 404. How should I make it like a post or page without manually overriding variables $wp_query, e.g. doing $wp_query->is_page = true;

I’m looking for an elegant way to do this.

EDIT:
I’ve added

    global $wp_query;
    $wp_query->parse_query("/");

After status_header, and now the page is displayed the way I wanted. Thanks.