Hi! I have a Mecool Ki Pro box and managed to run CoreElec itself also from nand without deleting Android! Simply assign the label COREELEC to /dev/system, STORAGE to /dev/data, mount /dev/system and copy the content of /flash to it with cp -a /flash/* <mountpoint>
Then, the minimum files to have in the root of any SD/flash drive with a FAT16/32 partition of almost any size is aml_autoscript, dtb.img and kernel.img, with that the box load only the kernel from the SD/flash drive and the rest is loaded from /dev/system. I can see the Android system files with a "ls /flash" and of course the data with "ls /storage".
For LE obviously change the above to LIBREELEC for /dev/system, and LIBREELEC_DISK or STORAGE for /dev/data, depending of the LE version as wrxtasy stated in the main message.
With this it would be possible even to make scripts/addons that takes advantage of this method and make backups of Android apk's or put any APK that will be installed on next Android boot xD and from the Android's point of view, I don't think it care of having the SYSTEM file on it's system root, unless you apply an update and that update formats /system or remove the files... in that case will only have to copy again the CE/LE files to /flash and maybe assign again the label, even I think that can create a flashable ZIP for install/recover it from recovery.
One of the problems I see is what would happen when applying an CE/LE tar update, I'll try hehe, if CE/LE only replaces SYSTEM, kernel.img without formatting /flash it would be ok Edit: tested it and it don't format /flash, so no problem the other only problem is what I noticed here some months ago, NEVER do a hard-reset with /dev/data shared as LE/CE will wipe all /storage data, so not only LE/CE will reset but also Android xD
Edit 2: it's ok to do a soft-reset, as it only remove config files, addons, etc. without deleting videos, downloads... so it also don't remove Android files