A plugin really is going to be difficult to get working. OpenVPN needs to run in the host OS, not a jail. I have a little write-up on how to get openvpn running on 8.2 on my blog.
http://joepaetzel.wordpress.com/2012/07/24/openvpn-on-freenas-8-2/
There are several issues here. The point of a Filer is data security and retention. The data safety must come first in any NAS. Adding another service net accessible service to the host is another vulnerability to that host. In the commercial NAS world, this is not acceptable and far from best practice. Personally the only reason I even consider FN is solely because of the jail implementation and I'm glad to see 8.2 simplify them as a "plugin" type system.
Had I even known before that Open VPN was installed in the first place, I would have never used it, and I might even consider stripping it out now. Else firing up my NetApp and just bite paying for the energy bill.
There are several ways to implement OpenVPN from within the jail, and that jail can be bound to an ethernet port. BSD is pretty good at this but it does have room for improvement. Specially MAC addressing for the jail for layer one. But this would not affect openVPN either.
I don't know about you, but while I host allot of my media on my filer, MP3s, some movies, etc. I have some very personal material on there as well in the form of personal paperwork, family photos, etc, and confidential work data that could be detrimental to companies or customers if it should be compromised. The filers security is in the commercial world first and for most. Most commercial gear will even take itself offline and shutdown within 24 after any problems are detected if they are not fixed within that time window, just to insure data is not lost.
Linksys makes some nice VPN Routers, I might even have an extra small office commercial version laying about.