I just tried this today, and noticed the same thing. Looking forward to having it in the next OPDS update!
higlider, the main reason I like azw3 more than mobi is because azw3 is "tweakable" by Calibre, where mobi is not. Sometimes books have extra pages in them I don't want, or if I've converted them more than once, they might have duplicate metadata pages in the beginning. I used to have to convert my mobi files to epub files, tweak them, and convert them back to put them on my Kindle. Now I can just convert all my books to azw3 and fiddle with them whenever.