General :: How To Write Files Into The Directory

I'd like to know if there's a way to write files into the main directory just before the system shutdown, something like this:

End all processes (-> unmount disks?) -> mount /sdcard rw and / rw -> copy files from /sdcard to / -> unmount disks -> shutdown/reboot

Same with boot image (flash, in this case, and not copy / mod files)... is it possible? Or do I require to boot into recovery for these operations? I know I can remount as rw my system directory via ADB

Also, is it possible to do the same thing with the build.prop file? This, or during startup (before mounting partitions with standard permissions).

Of course, this having root (but not a special kernel to enable this kind of "before mounting" thing).