I tried Coolreader in a rooted Sony and you can export the whole selected text. I selected one whole page and it exported the whole text without limits.
I don't know if it will be able to export (memory issue) all the marked text in a book of 500 pages, but you can definitely export all the notes and text every 20 or 30 pages, for instance, just in case it doesn't manage to cope with a lot of pages.
The highlight in last versions of Coolreader doesn't allow for transparency. That's to say: it's black in the Sony screen and you can't read the marked text. In prior versions (for instance 22.214.171.124) there's no highlighted, just underlined text. So with this version you can export the text and you can read on screen the selected text.
I think it would be better to highlight properly the text, but I don't know if there's a way.