I wonder if it would be possible to implement that feature on an Android device that allowed you to flash your own custom appsboot/lk? I've only spent a little bit of time in LK stuff, but it seemed to me that that was where that functionality lives.

From leaked LG LK code it shows that's definitely where their bootloader unlock keys lived [at one point a couple of years ago].

Since the Librem device is a different beast, though, I wonder what their bootstack looks like? Super curious now!