Yup. You just access a shared drive over a WAN link, e.g. an NFS shared volume. No reason why this can't be done over VPN.
Of course, you'd be nuts to do this with a backup. If you have the shared drive mounted read-write, then any malware on the system could toast the backup.
That's why you don't backup to a network share. You either use a different type of protocol, e.g. rsync, FTP, etc. Ideally, you would use a snapshotting file system. Trigger a snapshot, and then rsync or FTP the snapshots off to the backup server. Alternatively, you run the backup software on the backup server, and on your production machine you expose the data drive or snapshot files as a read-only share. The backup server connects to the share read-only and copies all the data off.
For the truly paranoid, the backup server can be firewalled off, except for when a backup is due. When a backup is due a cron job on the backup system temporarily lowers the firewall, initiates the backup, and when the backup is complete the firewall is brought back up. Taking this to extremes, you can use multiple backup servers which operate in rotation, but where only one has an open path through the firewall at any time.