<?xml version="1.0"?>
<config>
    <modules>
        <Bileamara_SpecialCollection>
            <version>0.0.1</version>
        </Bileamara_SpecialCollection>
    </modules>
    <global>
        <blocks>
            <specialcollection>
                <class>Bileamara_SpecialCollection_Block</class>
            </specialcollection>
        </blocks>
        <models>
            <specialcollection>
                <class>Bileamara_SpecialCollection_Model</class>
            </specialcollection>
        </models>
    </global>
</config>

<?php
class Bileamara_SpecialCollection_Block_Magazzino extends Mage_Catalog_Block_Product_List
{
    public function getLayer()
    {
        $layer = Mage::registry('current_layer');
        if ($layer) {
            return $layer;
        }
        return Mage::getSingleton('specialcollection/magazzino');
    }
}

<?php
class Bileamara_SpecialCollection_Model_Magazzino extends Mage_Catalog_Model_Layer
{
    public function prepareProductCollection($collection)
    {
        $collection
            ->addAttributeToSelect(Mage::getSingleton('catalog/config')->getProductAttributes())
            ->addAttributeToFilter('real_stock','1')
            ->addMinimalPrice()
            ->addFinalPrice()
            ->addTaxPercents()
            ->addUrlRewrite($this->getCurrentCategory()->getId());

        Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($collection);
        Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($collection);

        return $this;
    }
}