You can do by rewrite class  `Mage_Catalog_Block_Product_List_Toolbar`.

at `setCollection()` add this code:

    ->setOrder('name','DESC')->setOrder('name','DESC')

Rewrite class
-------------

    <?php
    class [ModuleNameSpace]_[ModuleName]_Block_Catalog_Product_List_Toolbar extends Mage_Catalog_Block_Product_List_Toolbar
    {
    	    public function setCollection($collection)
        {
            $this->_collection = $collection;
    
            $this->_collection->setCurPage($this->getCurrentPage());
    
            // we need to set pagination only if passed value integer and more that 0
            $limit = (int)$this->getLimit();
            if ($limit) {
                $this->_collection->setPageSize($limit);
            }
            if ($this->getCurrentOrder()) {
                $this->_collection->setOrder($this->getCurrentOrder(), $this->getCurrentDirection());
            }
    		// add this 
    		$this->_collection->setOrder('name','DESC')->setOrder('name','DESC');
            return $this;
        }
    }
    			

Rewrite code at config.xml
--------------------------

  

    <global>
    	<blocks>
    		[.....]
    			<catalog>
    				<rewrite>
    					<product_list_toolbar>[ModuleNameSpace]_[ModuleName]_Block_Catalog_Product_List_Toolbar</product_list_toolbar>
    				</rewrite>
    			</catalog>
    		[.....]			
    	</blocks>
      </global>