Thanks for this hack! It works great, but as mentioned some books have full-justification hard-coded, and this toggle won't override that
I'm using a K3 with Kindle OS 3.1.
For people where this didn't seem to work, I had the same problem, but that was because I didn't IMMEDIATELY restart the Kindle as indicated in the instructions. First I tried to use the switch, that seems to reset the config file so you lose your changes (as montalex mentioned above)
So it works for me. I sent a note to kindle-feedback asking why this wasn't just plain available and why we have to tap-dance to bend it to our will!