[onlynone]

You're right, it wasn't that root could mount the sparse images, but you can set a master password which "is a 'safety net' password, it lets you unlock any FileVault account on this computer". So you could use that to smartly backup any/all user FileVaults.

Of course any non-FileVault password protected images wouldn't benefit from this, which is why you could still have the option to manually specify the password to any images you want intelligently backed up.