Kopia does not let you pipe into it without intermediate files, or at least not in a friendly way I could figure out.
Kopia seems quite designed around the backup server being the place where the config is set. IMO it's harder to configure if you have your own declarative config tooling (like Ansible, NixOS, etc)
The nicest way I found to ignore files declaratively is to configure a ignore-file pattern in the repository policy and then declaratively create `.kopiaignore` files.
The Kopia server is convenient if you have a server at home and want to back up laptops to it, with them being in a shared deduplicated repository without having to trust the clients not to get pwned and wipe out the backups. However I'm not aware of a nice way to then transfer those backups to aggregate them into a bigger repository. (So I just rsync them off-site and don't worry about getting deduplication with other backups) Also learnt the hard way that you can't set a bandwidth limit when backing up to the kopia server, which can tank wifi performance on laptops during the backup.
I am thinking of switching to Restic as it seems like it is more suitable for people who want a tool at the complexity level of tar/git CLI with declarative config, rather than something imperatively configured.