Sorry I just noticed this reply today. Being in America I am spoiled to expect free internet in hotels, but I know that's not the norm in a lot of places, so I appreciate you going out of your way to reply while on your trip.

The /tmp directory is mounted just before my boot-shell code executes in /sbin/preinit if you use my exact code. I don't think root file system is read-only either, because I've edited /sbin/preinit when inside the shell launched by /sbin/preinit... (Though I think the boot process errors out right after the file save, probably something to do with it trying to execute the file's code but then having the file changed right under it.) Whether or not everything in the root file system is mounted though, or any parts haven't been 'made' yet (if there's anything that's in /tmp or anywhere else that's needed for the current busybox-power but that gets dynamically made/remade/updated later in the boot process).

So, in other words, way out of my league, but there is a "mount tmpfs [...] /tmp" line right before my code in /sbin/preinit. Either way, your latest reply to me by PM today is promising because it suggests this problem won't be relevant as soon as you implement history buffering inside busybox itself instead of the original 'use a temporary file' method.