Here it uses btrfs on top of mdraid, no LVM.
Guess it depends ?
/dev/md2 on /volume1 type btrfs (rw,relatime,synoacl,nospace_cache,auto_reclaim_space,metadata_ratio=50,subvolid=257,subvol=/@syno)
Since Duplicati allows you to run scripts before and after backup, one could mount the last btrfs snapshot and backup that, something like this could work.
subvolid=btrfs sub list /volume1 |grep "top level $ID_of_the_share's_btrfs_subvolume" |tail -n1 |grep -oP 'ID \K\w+'
mount -o subvolume=$subvolid /dev/$path_to_device /tmp/backup_source
Restoring would require mounting the live subvolume instead of the snapshot subvolume there, and you will need to unmount /tmp/backup_source after backup or restoration.
But this is untested, only theorical.
No it won’t, but the path you are supposed to use wouldn’t too as far as I am aware of.