Zotero's Export Library feature supports several XML formats, including TEI, which i'm using with Oxygen. However as far as i've been able to determine one has to export a whole library. Of course, one can make new libraries in Zotero and delete them, however this is a pain. Such a pain that my co-author and i have simply continued handcoding references for the book we're writing directly in Oxygen. (There's not too much overlap between the references in our book and my personal existing reference library, which is in Bookends, anyway, so there's not much incentive to switch to Zotero for that function.)

Am interested to hear about other solutions though, for this and future work.