Self answering my question.
Find the answer here:
https://www.mobileread.com/forums/sho...d.php?t=187054
In the hack posted in the above thread, the default configuration file allows using the Left / Right buttons for the following apps (tested): Reader (built-in), Browser (built-in), Aldiko Reader, Amazon Kindle for Android, FBReader, InstaFetch Pro, Google Reader, Google Books (old 1.x versions), CoolReader.
Credits are due to the originator and all contributors to the above thread.
Hope this helps others too!