First I tried building an SPK package for DSM7 which turned out to be quite a hustle (https://github.com/adn77/spksrc/commit/899720af3eacc469c18bc3650767c48e5ac7c72e) as there doesn’t seem to be a way to run anything as root in DSM7.
Since I am used to do a granular restore of content in disk images I wanted to get guestmount working as well… the dependencies are truly insane! It runs a QEMU(!!!) no way would I be able to package that nor cross-compile (well I am on denverton 
). That’s probably why the --mount-vhd configure option was not selected in @josef109’s package.
So my next try was Urbackup in Docker which installed very smoothly. The followed hint about the SYS_ADMIN capability.
Debugging the BTRFS snapshot issue I found that one can use the /sbin/btrfs binary in the container /bin/btrfs.
To do that in purely in the UI you have to first copy /sbin/btrfs to a location below homes (e.g. homes/urbackup/btrfs). Next you can map it as “File” to “/bin/btrfs” (note the directory, it’s not /sbin!)
With so much positiveness I tried installing libguestfs-tools into the container (build my own image based on uroni/urbackup:latest). I extracted the Linux kernel from the DSM_*_42218.pat and mounted it as /boot/vmlinuz-4.4.180±x86_64
The modules are mounted as /lib/modules:/lib/modules/4.4.180+
I tried mapping loop and fuse devices but ending up to be running the container “–privileged”.
… but still the guestmount just doesn’t work 
The command guestmount -r -n --format=raw -a /dev/loop0 -o kernel_cache -o allow_root -m /dev/sda /volume1/homes/urbackup/backups/mnt just doesn’t return.
Is there any sane way to get the guestmount working in Synology?