Is there a way to use an account for FileVault2 volume-decryption without logging it on?
I'm not aware of the key-combination.
However, here's how I did this on Mavericks (10.9.2):


Click System Preferences
Click Users &amp; Groups
If the Lock icon is closed, click the lock icon and authenticate
Click Login Options
Locate Network Account Server and click Edit (if there is a network server) or Join (if there is not)
Click Open Directory Utility
If the Lock icon is closed, click the lock icon and authenticate
Click Directory Editor icon
Search for the user you want to restrict
Click the user name
Expand Authentication Authority
Click the + - to the right of Authentication Authority, not the one at the bottom of the window!
Change new value to ;UserDisabled;
Click Save


The user can unlock the FileVault, but cannot login.