I have tested the ‘mount --bind /source.directory /var/lib/snapd/snaps’ method with Ubuntu 14.04.5 LTS on VirtualBox.
Snap then throwed the ‘Invalid cross-device link’ error under some circumstances.
Binding the whole parent directory ‘/var/lib/snapd’ instead of only the subdirectory '/var/lib/snapd/snaps’ worked afterwards.
If you suffer ‘broken’ snaps, see All snaps showing as ‘broken’