Custom feed tamper plugin to convert an image GUID into a URI
I came up with a solution, simply using a DB query to get what I want. Here is the callback for the tamper module. Hopefully someone will find a better solution, if it's possible.

function myfeedsmodule_tamper_imagekey_callback($result, $item_key, $element_key, &amp;$field, $settings) {
  $field = trim($field);
  if ($field == 'NULL') {
      $field = NULL;
      return;
  }
  $result = db_select('feeds_item', 'f')
          -&gt;fields('f', array('entity_id'))
          -&gt;condition('guid', $field, '=')
          -&gt;condition('entity_type', 'file', '=')
          -&gt;condition('id', 'my_image_importer', '=')
          -&gt;execute()
          -&gt;fetchField();
  if (!$result) {
      $field = NULL;
      return;
  }
  $entities = entity_load('file', array($result));
  if (!$entities) {
      $field = NULL;
      return;
  }
  $image = current($entities);
  $field = $image-&gt;uri;
}