I think I should explain a few things before I answer...
1. The ZFS Encryption that is being implemented in 8.3.1 is NOT the same ZFS encryption that is part of ZFS v30. FreeNAS' encryption will be disk-wide where ZFS's encryption is implemented in the I/O pipeline during compressions, checksumming and deduplication.
2. FreeNAS is based on FreeBSD, not the other way around. Generally stuff rolls down into FreeNAS and not up(with a few exceptions). If you are fluent in FreeBSD there is no reason why you couldn't implement FreeNAS' form of encryption right now(or even a year ago). Of course, FreeNAS would NOT be able to use the encrypted disks if you encrypted them with FreeBSD and then tried to use them with FreeNAS(at least.. not without you doing all of the legwork to make it work properly). This also has the drawback that if you encrypt your disks with FreeNAS and then switch to something like NAS4Free you cannot just import the zpool and go.
I wouldn't expect it to be ported back to FreeBSD because the port to FreeNAS is basically from FreeBSD and then the GUI slapped on top to make it easy for users like you and me.
So I think to answer your question in the way you are asking is to say "no".
I'm very interested to see how FreeNAS' implementation of encryption will work long-term. ZFS encryption(ZFS v30) is not likely to be implemented anytime soon since Oracle has not released that source code. It is also quite possible that if ZFS encryption is ever added to FreeBSD in any form except from Oracle's source code it is likely to not be compatible with Oracle's implementation.
Yes, the waters are very muddy right now. If Oracle hadn't decided to stop releasing future ZFS versions after v28 it is very possible that FreeBSD and FreeNAS would be using a newer version of ZFS than v28. The whole thing with Oracle closing off future versions of ZFS is causing alot of grief for alot of developers and users that want the newer versions. I can understand Oracle doing this(even though I don't like it or agree with it) because they are making a lot of money from companies needing encryption with ZFS. As more and more people and projects(like FreeNAS) start developing their own solutions you can bet Oracle isn't too happy about this. They may eventually release some of what will be their "older versions" of ZFS when it is no longer profitable for them to do so. But I wouldn't count on it.
