This was the first idea. I cannot explain very well why this does not work. But I think the issue is that the borg mount magic lives inside the container so the filesystem cannot be seen from the host. You can mount an empty directory and copy the files you want to access from the host into it. Problem with that is that you are stuck with the tooling provided by the container.
My setup is pretty much option 1, I have no issues with it. You can easly mount NFS shares as docker volumes (I m docking that for jellyfin and nextclould) but you need to get the permissons right. But I am no expert, just a hobbiest not smart enough for a better solution :)