When adding a share in the AFP services webpage, if you select Time Machine, that option is ignored. I would expect based on the netatalk documentation to see "tm" added to options for the share that Time Machine was selected for. However, that option is not there and furthermore, if I manually edit the /var/etc/AppleVolumes.default file to add in the "tm" option, that option is removed if I try to do any updates in the web gui. It is also removed if I restart the FreeNAS box.
If there is more information that is needed, please let me know.