I decided to go on working on it, since there aren't that many ebook accessibility tools especially on Linux. It's not really ready for the Plugin Index yet, since there are a few issues that still need resoving.
For example, ACE doesn't flag missing cover image descriptions, but is it because cover images don't need an accessible description or because it doesn't check svg-wrapped images? At least some sources say that you should describe cover images too, though just a simple "Cover of [book] by [author]" is sufficient. Currently missing cover image description is a fail condition, but I'm wondering if it should just be informative condition - but I haven't been able to find a definitive answer whether cover images need descriptions or not.
But even so, I can now quite easily convert a plain epub into an accessible epub that passes Epubcheck and ACE checks (other than metadata, as that cannot be automated) in about 15-30 minutes, without having to edit semantic tags by hand.
The source is available at
CodeFloe if someone wants to take a look.