This new filter provides the flexibility to not have to write a custom ZF filter when filtering is already available from another function.

class Zend_Filter_Callback implements Zend_Filter_Interface {
    /**
     * The callback function/method.
     *
     * @var callback
     */
    protected $_callback;

    /**
     * Additional parameters to send to the callback function/method.
     *
     * @var array
     */
    protected $_params = array();

    /**
     * Instantiates the callback filter.
     *
     * @param   $callback   The callback function/method.
     * @param   $params     Additional parameters to be sent to the callback function/method.
     */
    public function __construct($callback, array $params = array()) {
        $this->setCallback($callback)->_params = $params;
    }

    /**
     * Sets the callback function/method.
     *
     * @param   callback    $callback
     *
     * @return  Zend_Filter_Callback
     */
    public function setCallback($callback) {
        if (!is_callable($callback)) {
            throw new Zend_Filter_Exception('Invalid callback');
        }

        $this->_callback = $callback;

        return $this;
    }

    /**
     * Calls the callback function/method.
     *
     * @param   mixed   $value
     *
     * @return  mixed
     */
    public function filter($value) {
        $params = array_merge(array($value), $this->_params);

        return call_user_func_array($this->_callback, $params);
    }
}