CardDAV was accessible, but it was not documented. Some Googling could get you some archived mailing list threads where people posted the 'secret' URLs[1]. Even then, I couldn't get responses to all of the requests defined in the RFC. I remember getting a listing of supported request types from the server, but not all of them worked. Also "All Contacts" was not accessible. "My Contacts" was the only address book accessible to CardDAV.
For reference, I've put the stuff I was playing around with on Github: https://github.com/bsandrow/scratchpad-google-carddav
[1] I say secret because those mailing list posts were the only references I could find to them, so I don't know how they were obtained.