How can I set a dynamic value for post_updated_messages based on return value of post_updated?

I believe you can get the same result what you are looking for with filter hook post_updated_messages. You just need the post id for the confirmation thing that you are doing which you can do over here as well using $_GET['post'] which contains your post id.

Here is function which will do the thing.

function your_modified_message( $messages ){

    if(isset($_GET['post'])){

        $post_id = $_GET['post'];

        // get your response for respective post_id 
        $response = ping_other_service_and_get_confirmation($post_id);
        syslog(LOG_INFO, 'Response:' . $response['body']);

        // get your information concatenated with update post message in next line
        $messages['post'][1] = $messages['post'][1] . '<br>' . $response['body'];

        return $messages;

    } else {

        return $messages;

    }

}
// using filter hook to modify your message
add_filter( 'post_updated_messages', 'your_modified_message',10,1);

Hope this will work for you.

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