Filter by Web SubCategory?

How can modify the #PFS element to display filters for Web Subcategories? So when a customer arrives to a category, there are filters to narrow down the products by subcategories?

The way I do this is in Sync, map a custom text field to nitroasl_pamtable.pam_subcategory. Then in WSM create a new search group that gets its data from that custom text field. Its indirect and it uses a precious custom text field, but it works.