A possible useful feature for autocomplete

I made a form with a customized autocomplete field. I wanted it to only get results for books, so I found the part of the asin.module that decided this around line 396:

/**
 * Autocomplete callback for the asin_autocomplete widget.
 */
function asin_autocomplete_callback($string = '') {
  $items = $matches = array();
  // Search Amazon.
  $parameters = array(
    'ResponseGroup' => 'Small',
    'SearchIndex' => 'Blended',
    'Keywords' => urlencode($string)
  );
  $results = amazon_http_request('ItemSearch', $parameters);
  // Process the results.
  foreach($results->Items->Item as $xml) {
    $items[(string) $xml->ASIN] = (string) $xml->ItemAttributes->Title . ' (' . $xml->ItemAttributes->ProductGroup . ')';
  }
  // Create our response.
  foreach ($items as $asin => $title) {
    // Add a class wrapper for a few required CSS overrides.
    $matches[$title . ' [asin:' . $asin . ']'] = '<div class="reference-autocomplete">'. $title . '</div>';
  }
  drupal_json($matches);
}

All I did was change mine to:

/**
 * Autocomplete callback for the asin_autocomplete widget.
 */
function asin_autocomplete_callback($string = '') {
  $items = $matches = array();
  // Search Amazon.
  $parameters = array(
    'ResponseGroup' => 'Medium',
    'SearchIndex' => 'Books',
    'Keywords' => urlencode($string)
  );
  $results = amazon_http_request('ItemSearch', $parameters);
  // Process the results.
  foreach($results->Items->Item as $xml) {
    $items[(string) $xml->ASIN] = (string) $xml->ItemAttributes->Title . ' (' . $xml->ItemAttributes->ProductGroup . ')';
  }
  // Create our response.
  foreach ($items as $asin => $title) {
    // Add a class wrapper for a few required CSS overrides.
    $matches[$title . ' [asin:' . $asin . ']'] = '<div class="reference-autocomplete">'. $title . '</div>';
  }
  drupal_json($matches);
}

It might help others if somehow that could be set somehow when setting up the field on a form.