It is unlikely.
A third party app won't be able to open books in place. You would click on the widget, on a book that you already opened, and there're chances you won't get that book open in KOReader.
This will be worse when we address
https://github.com/koreader/koreader/issues/9787. At that point clicking on that widget will duplicate the book on a cache dir that doesn't inherit previous metadata unless you always started the book from the widget, which won't be possible unless the widget does media management on its own.
OTOH, while KO isn't a regular android app, we have access to the entire sdk and android facilities
outside MainActivity. So the effort to write a standalone widget or a widget as part of the program is mostly the same.
Anyways, not interested. But I wouldn't mind to integrate one on the program if somebody steps in and does the bulk of the job.