No problem. Turned out it was fairly simple to extract content.opf
, add a few lines (as below), and update the archive via zip.
<meta name="calibre:series" content="Series"/>
<meta name="calibre:series_index" content="#"/>
<meta name="cover" content="cover.jpg"/>
<meta name="Sigil version" content="0.2.3"/>
Adding the same meta tags through Sigil would save a step, but even doing it this way, the *.epub still validates... so why should I complain?