Вывод дополнительного сообщения после публикации поста

Привет, это можно делать через фильтр post_updated_messages. Будет примерно так:

add_filter('post_updated_messages', function($messages){
	$messages['post'][6] .= ' Дополнительное сообщение.'; 

	return $messages;
});

Список всех сообщений в массиве $messages:

Array
(
	[post] => Array
		(
			[0] => 
			[1] => Post updated. <a href="http://example.com/foo/">View post</a>
			[2] => Custom field updated.
			[3] => Custom field deleted.
			[4] => Post updated.
			[5] => 
			[6] => Post published. <a href="http://example.com/foo/">View post</a>
			[7] => Post saved.
			[8] => Post submitted. <a target="_blank" href="http://example.com/foo/?preview=true">Preview post</a>
			[9] => Post scheduled for: <strong>May 28, 2016 @ 03:20</strong>. <a target="_blank" href="http://example.com/foo/">Preview post</a>
			[10] => Post draft updated. <a target="_blank" href="http://example.com/foo/?preview=true">Preview post</a>
		)

	[page] => Array
		(
			[0] => 
			[1] => Page updated. <a href="http://example.com/foo/">View page</a>
			[2] => Custom field updated.
			[3] => Custom field deleted.
			[4] => Page updated.
			[5] => 
			[6] => Page published. <a href="http://example.com/foo/">View page</a>
			[7] => Page saved.
			[8] => Page submitted. <a target="_blank" href="http://example.com/foo/?preview=true">Preview page</a>
			[9] => Page scheduled for: <strong>May 28, 2016 @ 03:20</strong>. <a target="_blank" href="http://example.com/foo/">Preview page</a>
			[10] => Page draft updated. <a target="_blank" href="http://example.com/foo/?preview=true">Preview page</a>
		)

	[attachment] => Array
		(
			[1] => Media file updated.
			[2] => Media file updated.
			[3] => Media file updated.
			[4] => Media file updated.
			[5] => Media file updated.
			[6] => Media file updated.
			[7] => Media file updated.
			[8] => Media file updated.
			[9] => Media file updated.
			[10] => Media file updated.
		)

)

Обновлено: описал этот хук: post_updated_messages