ContentFinder Extension

I am adding a custom tab to the content finder in our CQ5 project. One of the requirements is to add tag lists from selected namespaces as a filter criteria. The expectation is to display the tags in the standard tab picker. Added the following in the items list section of my contentfinder extension file in /apps/wcm/extensions/contentfinder, with no luck:

{"xtype":"tags", "id":"some_id", "namespaces":["/etc/tags/mynamespace1",...]}

I also tried the xtype "optionspredicate" but still no data gets loaded in the CF.

Any insight to this is greatly appreciated.