OverlayFS and external SD card
This isn't a real answer to your question, but it is some hopefully useful information:

according to https://wiki.archlinux.org/index.php/Overlay_filesystem

"Overlayfs has been in the linux kernel since 3.18"

according to the answer and the comments here >>Which Android runs which Linux kernel?&lt;&lt;

linux kernel >3.18 seems rare in Android. If your kernel is >=3.18 you may be able to use some terminal commands to manually mount it how you want, I think your best bet would be looking up linux commands for using overlayfs.