Add to functions.php

function override_mce_options($initArray) {
    $opts = '*[*]';
    $initArray['valid_elements'] = $opts;
    $initArray['extended_valid_elements'] = $opts;
    return $initArray;
} 
add_filter('tiny_mce_before_init', 'override_mce_options');

A little more specific – allow empty tags if they have an id, name, class or style attribute:

function override_mce_options($initArray) {
  $opts = '*[id|name|class|style]';
  $initArray['valid_elements'] .= ',' . $opts;
  $initArray['extended_valid_elements'] .= ',' . $opts;
  return $initArray;
}
add_filter('tiny_mce_before_init', 'override_mce_options');