You'd need a patched initrd for that (c.f., @katadelos' previous post on that front

) (and explicitly flag the package with the right key number but our actual JB key, because I don't think the updater even supports the dev key index).
All of that *should* be doable via KindleTool, but, obviously, has never been tested (to my knowledge)

.
Or @yifanlu's old FW-3.x-on-DXg posts/scripts, which might have been doing something of the sort way back in the day

.