This part, I think, might be better requested from Lexcycle/Stanza. Looked at the epub files Calibre generates and the opf contains the following tags:
A separate entry for "Series" on the main library page is better than cluttering up the "Subjects" entry. I think it would be simple enough for Lexcycle to parse this information and add a separate category. Will suggest it on their website.
Is the <series> tag standard in epub or is it just something html2epub adds?
Oh yeah, itimpi, I hope you don't mind but I'm currently playing around with server.py trying to figure out how to integrate hierarchical listings in Calibre. Not sure if anything will come of it. I'm somewhat hopeless when it comes to programming.