How do I add an admin notice within javascript in the admin interface for posts?

Look at F:\sites\wp-git\wp-admin\js\common.js:

$('div.updated, div.error').not('.below-h2, .inline').insertAfter( $('div.wrap h2:first') );

If your notice has the class updated or error, it will be set after the first h2 automatically. When you create custom messages, make sure you are using one of these classes. Then you can just append them to body, and WordPress will move them to the proper place.