When you try to set the other category on the layer singleton from a template, it's too late as all the treatments were already applied.
What you can do is to copy the file app/code/core/Mage/CatalogSearch/Model/Layer.php
into app/code/local/Mage/CatalogSearch/Model/
and to add a modified version of the base Mage_Catalog_Model_Layer::getCurrentCategory()
method, looking like this :
public function getCurrentCategory()
{
$category = $this->getData('current_category');
if (is_null($category)) {
if ($category = Mage::registry('current_category')) {
$this->setData('current_category', $category);
}
else {
$category = Mage::getModel('catalog/category')->load(5); // Put here the ID of the category you want to use as basis for category filtering
$this->setData('current_category', $category);
}
}
return $category;
}