Because 'supplements' is a category (which changes) it's going to be more difficult to use a writing rule without hard coding. One possible solution is to use Mage::getModel('core/url_rewrite')
(example) to create a route for each (you may run into issues using this method)
Another solution would be to change the url format
/manufacturer/supplements/scivation.html
Then create a custom module
config.xml
<global>
<rewrite>
<fancy_url>
<from><![CDATA[/manufacturer\/(.*)/(.*)/]]></from>
<to><![CDATA[manufacturer/index/processroute/manufacturername/$2/]]></to>
<complete>1</complete>
</fancy_url>
<rewrite>
</global>
<frontend>
<routers>
<tagseo>
<use>standard</use>
<args>
<frontName>customcategory</frontName>
</args>
</tagseo>
</routers>
class MageIgniter_Manufacturer_IndexController extends Mage_Core_Controller_Front_Action
{
public function processRoute(){
print_r($requestUri = Mage::app()->getRequest()->getRequestUri()); //supplements/scivation.html
print_r($this->getRequest()->getParam('manufacturername')); // scivation.html
print_r($this->getRequest())
// do you custom logic here base on about request path explode('/', trim($requestUri,'/'))
//lookup the attribute_id from attribute name
//if attribute_id found then try
//$this->_forward(string $action, [string|null $controller = null], [string|null $module = null], [ $params = null])
// to the catalog page
// else $this->_forward('defaultNoRoute');
}
...
Read more @
Simple URL Rewrite Using Magento xml
Magento Router Url - Need Hyphnated Path Name
Magento V1.7 Grid View - Add manufacturer attribute to view