I think this feature would be useful, and your external-HDD use-case is one good example.
Another example: it would have been useful for me when I first started with restic, and needed to import my rsnapshot/hardlink-based backup into a new restic repo. Some info on the scripts I ended-up using can be found in response to this question. TLDR; to overcome the problem of paths I chose to symlink to each snapshot in turn and then pointed restic through the symlink. It would have been a lot more elegant to simply point restic to each snapshot path, while telling it to remove (or replace) the absolute location.
By the way, you could also apply the symlink workaround to your use-case.