At the moment per settings you have only one path for doing backup storage.
I think it would be good idea to have like multiple paths for the backups, where you can specify this for backup files only and another one for file images, it would be definitely great, because currently you need to have big hard drives to make it happen (storage for files and images).
So currently i do kind of “hack”:
I start the full or incremental file backup and put it on pause, then i log to my urbackup server and see new created directory for full file backup, then i create a symlink to another btrfs disk witht equal directory name and that’s it!
Back to urbackup client there i resume the backup process.
Positive: now i can backup files of my big disk.
Negative: i can’t rely on automatic of Urbackup server for my backups, i do for now my backups manually, it’s easy for me now, sure plan for future to buy more bigger SSD disks 
If you’re willing to sacrifice some reliability for convenience and automation, you may consider using RAID0/stripe.
As long as you can restore from the server, whatever works is a great solution!
If you say this works without problems, why not just symlink the base directory of the user instead of starting urbackup and THEN symlink every single backup. That means everything on that user is on another drive.
Should have the exact same effect but with the difference you don’t have to do it every single backup. xD
I could probably (if the symlink thing actually works) do the user symlink, but if I tried your method, pretty sure I cant just symlink a directory where a btrfs snapshot is mounted without consequences.
Or maybe I can, now I get unsure, maybe the snapshot moves to the symlink or something. It sounds completely wild to me.
Edit
Now that I think about it, why not just mount the other drive directly onto a userdirectory, therefore removing symlinks altogether?