Since you are talking about Samba I assume you have a Linux VPS acting as the centralized backup server. You can always create mountpoints for Samba shares in your /etc/fstab using the cifs mount type and putting the credentials in a file referenced in the options. For details see this Ubuntu wiki post on mounting Windows shares permanently
. At that point your site's root is basically a local path such as /mnt/site1. For all Solo cares, you have a local path. It doesn't need to know anything about the underlying filesystem and the OS won't tell it either. It is blissfully ignorant and works just fine :)
If your site servers are also Linux, however, I'd recommend using NFS instead of Samba for the network shares. It's a much faster protocol and you can definitely limit access only to your server. Please do not try using fish, sshfs or any such end-user-oriented solution; they are fine for the occasional file transfer but suffer horribly for performance when you're trying to access an entire site's files.