If a page does not exist, include a different page?

I managed to solve the problem. Here’s how to do it, in case anyone else ever needs this solution.

** Updated Mar 10 3:30am EST **

The previous function that I had here was displaying the page properly, but was returning a 404 error code. I’ve replaced the bad code example with a working one that does not return a 404.

function check_section_page( $posts ) {

    if (is_admin()) { return $posts; }

    global $wp, $wp_query;

    $requested_path = $wp->request;

    if ( $requested_path == "" ) { return $posts; }

    preg_match("/^([a-z\-]+)\//", $requested_path, $matches);

    if ( count($matches) > 1 ) {
        if ( get_page_by_path( $matches[1] ) ) { $section = $matches[1]; }
    }

    $wp_query->set("saleslink_section", $section);

    $this_page = get_page_by_path( $requested_path );
    $new_requested_path = preg_replace("/^(.*?)\//", "", $requested_path);
    $requested_page = get_page_by_path($new_requested_path);

    if ( $requested_page->ID and ! $this_page->ID ) {

        $posts = NULL;

        $posts[] = $requested_page;
        $wp_query->is_page = true;
        $wp_query->is_singular = true;
        $wp_query->is_home = false;
        $wp_query->is_archive = false;
        $wp_query->is_category = false;
        unset($wp_query->query["error"]);
        $wp_query->query_vars["error"]="";
        $wp_query->is_404=false;

    }

    return $posts;

}

remove_filter('template_redirect','redirect_canonical');
add_filter("the_posts", "check_section_page");

The remove_filter line will turn off the automatic-url-guessing-redirect behaviour of WordPress, when a requested URL does not exist and a similar one does and it redirects to the similar URL.