Check all WordPress Media Items not attached to a post/page.

gistfile1.js

/**
 * Must be viewing the media library of a WP install.
 * Media library must be in list view.
 * Will highlight checked items in yellow. PLEASE CHECK TO MAKE SURE THAT THOSE ARE THE ONES YOU WANT.
 */
var $ = jQuery;

$('#the-list')
  .children('tr')
  .filter(function(){
    var d = $(this).children().filter(':nth-child(5)');
    if(d.text().match(/Unattached/)) {
      return true;
    } else {
      return false;
    }
  })
  .css('background-color', 'yellow')
  .find('input[type=checkbox]')
  .prop('checked', true);