I think it’s unlikely that restic will support this as a first-class feature because, frankly, it’s out of scope.
Speaking generically, a backup tool is given a source and a destination and it makes sure the source exists in the destination in some useful way. You might say that tar and cp fit this description and could be classified as backup tools. To that, I say: yes, I agree. They are (in a way) backup tools. Crude, perhaps, but workable.
Gaining access to a remote system’s content is a different task better suited for a different tool, like NFS, sftp, or rclone. You can combine these with a backup tool to achieve what you want.
I do think it might make sense to allow specifying an rclone remote as a source, only because this makes the integration substantially simpler (rclone mount is no longer necessary). Anything beyond that is making restic responsible for too much, in my opinion.
Put another way, restic is not designed to be a complete enterprise backup solution, but you could certainly use it as part of one.