Plugin action rewrite rule - non_wp_rules

Question

The wp rewrite rule I added to my plugin is not working. Here's my setup:

Scenario

I'm trying to get the site's ads.txt to pull from my plugin's ads.txt. eg site.com/ads.txt pulls from wp-content/plugins/the-plugin/ads.txt - all done via serverside.

Plugin portion

add_action( 'init', 'wpse9870_init_external' );

function wpse9870_init_external()
{
    global $wp_rewrite;
    $plugin_url = plugins_url( 'ads.txt', __FILE__ );
    $plugin_url_sub = substr( $plugin_url, strlen( home_url() ) + 1 );
    // flush_rewrite_rules(true); // tried flushh ere
    $wp_rewrite->add_external_rule( '^ads.txt$', $plugin_url_sub ); // tried it with and without the caret (^)
    // flush_rewrite_rules(true); // also tried flush ere
}

Debug

WP_Rewrite Object {
...
    [non_wp_rules] => Array
    (
        [^ads.txt$] => wp-content/plugins/the-plugin/ads.txt
    )
...
}

Going to site.com/ads.txt results in a 404.

What else should I try?