How to add custom widget above admin_footer

As far as I know you have 2 options.

1. Using admin_footer or load-edit.php and position with absolute or fixed, or use JavaScript.

2. If you creating a completely new posts screen (not a default posts, pages, etc), you can extend WP_List_Table class, specifically the extra_tablenav function.

For example:

class My_Custom_Table extends WP_List_Table {

    function extra_tablenav( $which ) {

        if ( $which == "top" ){
            //The code that goes before the table is here
            echo "Hello, I'm before the table";
           }
        if ( $which == "bottom" ){
            //The code that goes after the table is there
            echo "Hi, I'm after the table";
           }
      }
}

As far as I can tell, extending this class can only work on a custom admin page, and not the current existing ones, as there are no hooks available. You can read more about it here: http://codex.wordpress.org/Class_Reference/WP_List_Table