I have fully implemented mutable torrents (BEP 46) in Transmission. When a torrent is created, you can set it to be "mutable", and you (and you alone) can add files to the torrent, remove them, modify them, or change their filenames. Other members of the swarm will be notified of the new change, and begin downloading that as well (if they use a client with mutable torrent support). For leechers, they can choose whether to allow mutability on a per-torrent basis, and only in the fashion that they prefer. They can even store every change (and seed those) too.

I have the macOS, Windows, cli, and web app working with this feature. I had a bit of a mixup with Gtk, so I don't have a Debian package for it, but it's buildable from source.

https://github.com/NoMoreNicksLeft/transmission

I would appreciate it if anyone wanted to test it. I'd like to think that the feature would be a big deal, even if my implementation of it's kinda crappy.