I have had some success in converting a KDF file produced by the Kindle Previewer 3.0 beta into a KFX file that can be sideloaded onto a Kindle device or app. I have attached a sample converted from
a public domain EPUB.
I have tried it on the Previewer (after changing the extension to .azw8), a Kindle Paperwhite II (5.7.2.1 firmware), Kindle for Android 4.21.0, and Kindle for iOS 4.17. As far as I can tell it functions properly on all of these. If anyone tries it and notices a problem please let me know.