Aggregating computed Entity property in a View field?

I tried to use views_handler_field_entity, but that doesn't work. It gives the error:

Warning: htmlspecialchars() expects parameter 1 to be string, object given in htmlspecialchars() (line 1545 of /var/www/html/drupal_dev/includes/bootstrap.inc)

What am I missing here? This is the code I put in to use it. What else does the handler need to work? Maybe there needs to be some information about the module that defines the entity? Maybe it's for handling a whole entity as a field, not just a property of an entity.

  $data['uc_order_products']['table']['entity type'] = 'uc_order_product';

  $data['uc_order_products']['total_price'] = array(
    'title' => t('Total price'),
    'help' => t('The price paid for all the products (price * quantity).'),
    'field' => array(
      'handler' => 'views_handler_field_entity',
      'click sortable' => TRUE,
      'float' => TRUE,
    ),
    'sort' => array(
      'handler' => 'views_handler_sort',
    ),
    'filter' => array(
      'handler' => 'views_handler_filter_numeric',
    ),
  );

Using this:

'handler' => 'entity_views_handler_field_numeric',

...almost works. It correctly gets the value (using the getter callback) in a regular Views. However, it doesn't work for aggregated fields. The query ends up with a "GROUP BY" on the entity ID, which means that the computed values can't be summed. Any idea how to fix that?

Does it have anything to do with entity_views_plugin_row_entity_view? I can find zero documentation on how to make a View use a plugin.