How to set input renderer on attribute for custom entity?

Question

I'm trying to use an input renderer for a text attribute in my custom entity, however it doesn't seem to be working.

I'm creating my ambassador entity in my setup file like this:

<?php
/* @var $installer Mage_Eav_Model_Entity_Setup */
$installer = $this;

$installer->startSetup();

$installer->createEntityTables($installer->getTable('ambassadors/ambassador'));

$installer->installEntities(array(
    'ambassador' => array(
        'entity_model' => 'ambassadors/ambassador',
        'table' => 'ambassadors/ambassador',
        'attributes' => array(
            'type'           => 'varchar',
            'input'          => 'text',
            'label'          => 'Featured Color',
            'required'       => false,
            'input_renderer' => 'ambassadors/adminhtml_helper_color'
        )
    )
));

$installer->endSetup();

The attribute is created and assigned to my entity but it renders like a normal text input in my edit form in the backend, I was looking at the Mage_Eav_Model_Entity_Setup class and it looks like input_renderer doesn't get passed in the _prepareValues method so I think this is part of the problem:

protected function _prepareValues($attr)
{
    $data = array(
        'backend_model'   => $this->_getValue($attr, 'backend'),
        'backend_type'    => $this->_getValue($attr, 'type', 'varchar'),
        'backend_table'   => $this->_getValue($attr, 'table'),
        'frontend_model'  => $this->_getValue($attr, 'frontend'),
        'frontend_input'  => $this->_getValue($attr, 'input', 'text'),
        'frontend_label'  => $this->_getValue($attr, 'label'),
        'frontend_class'  => $this->_getValue($attr, 'frontend_class'),
        'source_model'    => $this->_getValue($attr, 'source'),
        'is_required'     => $this->_getValue($attr, 'required', 1),
        'is_user_defined' => $this->_getValue($attr, 'user_defined', 0),
        'default_value'   => $this->_getValue($attr, 'default'),
        'is_unique'       => $this->_getValue($attr, 'unique', 0),
        'note'            => $this->_getValue($attr, 'note'),
        'is_global'       => $this->_getValue($attr, 'global', 1),
    );

    return $data;
}

The Mage_Catalog_Model_Resource_Setup does pass this attribute but it still doesn't work if I use that class instead, however it works fine if I assign the attribute to the catalog or category entities so I assume that setup class is linked to those entities in some way? Do I have to do something else to enable custom input renderers in my entity? Do I have to initialise it differently?