You’ll find that this won’t work the way you want it as LXD runs in its own mount namespace and can’t just access all host paths.
There are ways to do this still, but it’s quite manual and not something we’ll officially support. In fact, people doing things by hand and getting into a bad spot is exactly why we implemented storage.images_volume in the first place 
There’s nothing wrong with you defining a new dir storage pool, point it to what you want and then use volumes from that to back your images/backups though.