I have been watching and read through that thread for a few weeks and I did not see anything about it working on 10.4.3.Have you used this method yourself on the latest firmware?
I see nothing to suggest that the instructions for installing the 2nd bootloader would be any different on 10.4.3. The same instructions worked for the other KFs that received updates at the same time, and previous updates generally all had the same fixes included for each device. The only thing I can imagine that Amazon could do to prevent installation of the 2nd bootloader is to disable fastboot, but even then, it doesn't really "prevent" anything.
Then again, you never know. I may be totally wrong and it could cause a thermonuclear meltdown in your lap (or just brick your device).
That's the problem with not disabling OTA updates on a rooted stock device. When a new update gets pushed to your device, you're always taking a risk when trying to modifying it again. It's your device, and ultimately it's your responsibility if it gets bricked beyond repair in the process of doing so. If you think it's worth the risk, go ahead and try to install the 2nd bootloader and let everyone else know how it went. Otherwise, you'll have to wait around for someone braver than you to test it out and (hopefully) let you know.
