Let me have the ability to query a Wordpress blog for a listing of existing tags to ensure that they may be offered at that time that the new publish has been produced. Is possible through AtomPub with Wordpress? I am locating the documentation for Wordpress AtomPub support to become missing. I am wondering if this sounds like much more of a set limit on AtomPub than you are on the Wordpress implementation.

It appears Atom doesn't have a perception of "tags", only "categories". As AtomPub is dependant on it, it only identifies a request for categories.

The WordPress AtomPub implementation calls [cde] directly, which, obviously, defaults towards the get_categories() taxonomy. However, you are able to filter the arguments via category, and alter the get_categories_taxonomy argument for an array which includes taxonomy. You'd obviously only enable the hepa filter when there's an AtomPub request.

Whenever a publish is produced or up-to-date WordPress only compares the groups, so you'll have to add the non-category terms yourself. This can be done by connecting in to the post_tag and atompub_create_post hooks, which gets the WordPress publish ID and also the full entry the customer sent, including all terms.