没那么难，你可以用all_admin_notices-挂钩，显示在Post Updated/Saved, 等消息框。

包装在插件中：

<?php
! defined( \'ABSPATH\' ) AND exit;
/** Plugin Name: (#64933) »kaiser« Add post/page note */

function wpse64933_add_posttype_note()
{
    global $post, $pagenow;

    // Abort in certain conditions, based on the global $pagenow
    if ( ! in_array(
         $pagenow
        ,array(
             \'post-new.php\'
            ,\'post.php\'
         )
    ) )
        return;

    // Abort in certain conditions, based on the global $post
    if ( ! in_array(
         $post->post_type
        ,array(
             \'post\'
            ,\'page\'
         )
    ) )
        return;

    // You can use the global $post here
    echo \'<p>HELLO WORLD!</p>\';
}
add_action( \'all_admin_notices\', \'wpse64933_add_posttype_note\' );