Ok, I figured it out myself:
in wp-includes/functions-post.php
in function “user_can_edit_post” (that is on line 357) change $author_data->user_level > $post_author_data->user_level to $author_data->user_level >= $post_author_data->user_level
Good that the variables have speaking names! IMO good coding style..!
I am very interested in a solution of this problem too. And that option would be fine.. 🙂
