Disk encryption on HAMMER volumes

[PeerCorps Trust Fund]

Greetings!

Since encryption has come up in the lists lately I thought I might chime in with an additional question. 

What is the best way to encrypt single disks in a non-RAID HAMMER disk set-up?

There are four disks in the machine in question. Dragonfly is installed on a separate SSD. My interest lies in encrypting only two disks in the machine and have them be mounted with a password prompt at boot time (in similar fashion to the way in which the installer does it for ROOT). 

An example of what I am trying to do:

- SSD --(HAMMER ROOT, not encrypted)
-- HDD --(independent disk, encrypted)
-- HDD --(independent disk, encrypted)
-- HDD --(independent disk, not encrypted)
-- HDD --(independent disk, not encrypted)

Alternative suggestions for are above are also very welcome and appreciated.

-- 
Mike