I too have considered converting my (freeware) stand-alone program, GuteBook, into a calibre Plugin, though I lack Python programming skills AND any knowledge of the calibre API!!! (
My GuteBook program extensively uses calibre commandline utilities, but as a Plugin it would escape it's DOS roots, be "embedded" within the calibre GUI and thereby allow easier integration with my GUI and functionality.
However, a GuteBook plugin would not tweak/operate on existing ebooks, but rather would retrieve and create
ebooks from Project Gutenberg "on the fly" after applying many "clean-ups" and fixes as your eBookCleaner would do.
So, if you ever get this Plugin working, I may be knocking at your door to augment it for a GuteBook Plugin...
I too look forward to seeing this Plugin to written!