Hi, Kovid. Still working on getting some funding from my department for this project. The wheels here turn slowly, though I've been told it's forthcoming.
I've got a few links here that might be of use to you in development of html2epub.
The first has already been discussed here (re: filesize limits):
The second, regarding the ePubCheck utility is from the mobileread forum, so you're probably already familiar:
This third is on the Adobe devnet area. Several useful links on this page, but again, you may have already come across this:
This fourth one is a page with many useful links from the Idiotprogrammer (not to suggest you're an idiot!):
Thanks so much for your great tool, and I can't tell you how excited I am that you're working on adding html2epub support. Let me know if you need assistance beta testing the product as you develop.