I just came across this announcement:
https://archiveofourown.org/admin_posts/12212
I tried their sample EPUB and it now contains series, comments and tags. Whoohoo! Their old engine only had title, author and publisher, and the MOBI output didn't even have proper TOC nor chapter breaks.
As someone who often downloads directly from AO3 to reader and rarely accesses desktop Calibre GUI (ergo FanFicFare), this is a very welcome development.