That's why I would like the choise: USB key, password or both. Up to the user what to use.
I don't think they can just copy the FreeNas solution
I think FreeNas, just like QNAP and Synology, have a RAID storage pool in place and on that storage pool you can create a volume (encrypted or not).
Unraid works a bit different, I has a number of seperate disks and calculates the parity with the data on those disks to a parity disk (a bit like SnapRaid and some FlexRaid solutions do).
Therefore there is no "single volume" to encrypt.
What I think for Unraid a solution could be is encrypting one, multiple or all disks (user could choose), and mount those necrypted disk at boot-time with the encryption password/usb key.
Those mountpoints would then be part of the Unraid array (instead of the original partition mount), and after this the array would be started.
For the Unraid array itself it would then be completely transparant.
But I could be wrong in this :-)