Joining tables not working in the post editor page

The easy way in the admin is to do a 2nd custom query using the post_id from the post object.

function custom_posts_data( $posts, $query ) {
       global $wpdb;  
       if ( !count( $posts ) ) 
           return $posts;  

       while ( $posts as $post ) {        
           $query = "SELECT * FROM {$wpdb->prefix}my_plugin_table WHERE post_id={$post->ID}";
           $results = $wpdb->get_results( $query );
       }
       return $posts;
   }
   add_filter( 'the_posts', 'custom_posts_data' );

If you need to do the full joins have a look at scibu’s sortable columns example
or Mitcho’s orderby joins filter examples.