First, let's find out how Magento displays the filters in left hand navigation
1) Enable Template Path Hints:
and here is the result:
2) Let's take a look at app/design/frontend/base/default/template/catalog/layer/filter.phtml
(you should copy this file in your theme folder structure)
3) The block ($this in the template file is an instance of Mage_Catalog_Block_Layer_Filter_Attribute which extends Mage_Catalog_Block_Layer_Filter_Abstract
)
4) You can see that in Mage_Catalog_Block_Layer_Filter_Abstract
a getName() method exists, so you could rely on this function to identify when you attribute is displayed. Don't! The label can change and your code won't be useful any more.
Back to our template file, you can get the attribute_code (which is reliable)
//$this is instance of Mage_Catalog_Block_Layer_Filter_Attribute
$attributeModel = $this->getAttributeModel();
$attribute_code = $attributeModel->getAttributeCode();
5) On your template you can have a check based on attribute code, so you display either the standard list or your custom html code with a textarea instead of a huge list.