Union/overlay "filesystem" using LD_PRELOAD

We experimented with this for klik 9 years ago:
https://www.winehq.org/pipermail/wine-devel/2007-November/060627.html

Someone else seems to have had some success with this idea:
http://algoholic.eu/unionfs_by_intercept/

deb2snap seems to be going down that path, too:
https://github.com/mikix/deb2snap/blob/master/src/preload.c
Actually the whole deb2snap thing could be massaged into a deb2appimage thing relatively easily, it seems.
Discussion:
https://lists.ubuntu.com/archives/snappy-devel/2015-February/000282.html

user-union:
http://www.dwheeler.com/user-union/ (thanks for the hint @stsp)

Or we could use FUSE:
https://github.com/rpodgorny/unionfs-fuse

CARE/PRoot:
https://packages.debian.org/unstable/main/care
"PRoot is basically better fakeroot-ng plus fakechroot, or schroot minus root privilege"
http://www.slideshare.net/cvinc02/proot-improvedkernelcompat
https://github.com/proot-me/PRoot
Unmaintained?

cde:
`cde ls` creates an empty cde-package directory where I can *add* (not overwrite!) an AppDir, then run it with cde-exec appname. It does an overlay/union