Allow setting the UUID from the front-end

uuid-setter.php

<?php

/**
 * Implements hook_entity_property_info_alter().
 */
function mymodule_entity_property_info_alter(&$info) {
  // Add the uuid setter if not present.
  foreach (entity_get_info() as $entity_type => $entity_info) {
    if (
      !empty($entity_info['uuid'])
      && !empty($entity_info['entity keys']['uuid'])
      && !empty($info[$entity_type]['properties'][$entity_info['entity keys']['uuid']])
    ) {
      // This entity type supports a UUID.
      $uuid_info = &$info[$entity_type]['properties'][$entity_info['entity keys']['uuid']];
      $uuid_info['setter callback'] = 'entity_property_verbatim_set';
    }
  }
}