Connect the Kindle to your computer via USB an access the folder "system" (it is a hidden folder, blindly type "system" on windows explorer address bar and press enter, or configure windows to show hidden files).
Then access the "com.amazon.ebook.booklet.reader" subfolder and edit the "reader.pref" file (use an editor like
editpad pro that preserves unix LF line endings instead of windows notepad which will end lines with CR LF).
Set the undocumented option
ALLOW_JUSTIFICATION_CHANGE=true
Restart the Kindle (home, menu, settings, menu, restart).
Now, pressing Aa will show the new justification option.
Before/After:
