This can be configured by modifying the "com.pharos.signup.plist" located in the /Library/Preferences folder.  The key is named "Client Cache File Directory".

You can make the change by running this command in (you will be prompted for your password):

sudo defaults write /Library/Preferences/com.pharos.signup "Client Cache File Directory" "PATH"

Be sure to replace PATH with the correct path to your persistent storage area (e.g. the DeepFreeze thawspace).  You will need to restart the machine for this change to take effect.