Originally Posted by hawhill
Another suggestion: use unionfs-fuse to make COW overlays onto the root filesystem. FUSE is available and building unionfs-fuse should work fine. It's very similar to bind mounts, but it somewhat automates the copy-on-write path (but not in the first case presented next).
Sounds interesting but that would involve remounting the root partition late in the boot process. I don't think that's a hassle-free operation.
You can't do it before exporting US over USB because if things go wrong due to an error in one of the hacks the user wouldn't get chance to get rid of the hack by deleting its home dir.
I'd stick to bind-mounts, it's a tested, safe and lightweight solution.
COW is not that important here giving the fact that root fs modification/substitution is not desirable and should be reduced to a minimum.