Ive had to do something similar before.   It is painful, but doable.

* create the directory structure on the target. (find -exec...)

* create a symlink for every file on the target device, obviously pointing to the origin device. (Find)

File system should be useable.

*  progressively replace the symlinks with the actual file (find or script)

* watch out for actual symlinks...

Optionally replace the origin file with a symlink to the new file.

Teach your kids Science, or somebody else will :/

callsign: vk2vjb