I had the same issue... and discovered that if I locally mount a folder from the remote system, then rsnapshot will allow me to set the "snapshot_root" configuration parameter to point to it.
This then allows me to store my snapshots and my backup on the same remote server.
Mount a folder from the remote system
mount 192.168.0.2:/volume1/Backup /mnt/Backup
Set the "snapshot_root" configuration parameter to point the mounted folder
Set a rsnapshot backup point to backup a local folder remotely
backup /home/backup-source firstname.lastname@example.org:/volume1/Backup/backup-target/ rsync_short_args=-trvsz
Note: Although I could have simply designated the mounted remote folder instead as a backup destination, (see below), this would not have enabled me to utilise the Rsync server running on the remote target, (in this case a Synology NAS). I used shared keys to allow rsnapshot to access the remote backup target without a password
backup /home/backup-source /mnt/Backup/backup-target/ rsync_short_args=-trvsz