Filtering for battle pets that have been put into a cage seems to not work. Both when searching for the name of the pet ("Pandaren Water Spirit") or when searching for the negation of anything ("!qqqqqq", for example).

Apparently GetItemInfo() does not work with links to battlepets. Found a fix when looking for similar problems in other addons at http://www.wowace.com/addons/baggins_search/tickets/3-error-about-nil-item-name-with-caged-pets-in-bags/