Change your helper function such that it accepts a parameter, as shown:
In Zend_View_Helper_Navbar:
public function navbar($foo="")
{
$this->_view->bar = $foo;
return $this->_view->render('navbar.phtml');
}
Then, in navbar.phtml:
<?php echo $this->bar; ?>
This way, whatever parameter value passed to the helper function will be displayed in the navbar.phtml. After that, you can pass the parameter from your controller file as usual.
In your controller file:
$this->view->foo = "custom parameter";
In your view script, or layout.phtml, call the navbar helper passing the parameter:
<?php echo $this->navbar($this->foo);?>