The plugin (using QSvgRenderer and PyPdf) works on my machine, I'll be testing it on other platforms over the next couple of days. If all goes well, I'll publish the plugin early next week.
Currently, it's only capable of processing notes from (unscaled!) PDF books. I'm not quite sure what to do with EPUBs. First of all, I'd have to find out how EPUB scaling works on the reader. After that, I'd have to hook the plugin up to Calibre's EPUB conversion code in order to produce an output PDF.
Before I start digging through the Calibre source code looking for hints - which approach would you prefer:
1) convert both ebook and handwriting to PDF, merge into single PDF.
2) directly render handwriting on EPUB. I have no clue whatsoever how EPUB is structured, any clues?