this can be solved on the file system layer below.
on my laptop with btrfs this requires no special effort: simply copying the files and renaming them suffices. nautilus (gnome file manager) uses copy on write, and coreutils "cp" also defaults to "reflink" these days.
Doesn't work for torrents with RAR/ZIP files. The ref-link is also one-way. If I have existing content on my disk, and want to (partially) seed a torrent - can't do that easily.
I'd like a way to say: "Everything inside /public is public-domain content, seed it against any valid torrent if someone shows up with a valid hash"
If I have an arxiv backup, I can’t re-use the same for a arxiv-backup-torrent easily.
I get that disk space is cheap, but I see the allure of a content addressable system to solve for this.