Yes you can. Just have to do NFS mounting from within the jail. This would work any way any FreeBSD install would work, so you might want to Google how to do that on FreeBSD.
Do note that *if* you go this route and your machine that isn't running the Plex jail goes down your media will be unmounted in the jail and will not automatically remount. You will need to remount it yourself, from within the jail. Also Plex will happily delete the entries for the media that appears to have been deleted. So later when the NFS share is remounted it will go and download all the info again. If this isn't palatable then you will need to disable a setting in the Plex GUI. But the media on the NFS share will still "appear" to exist, but when you try to open one with Plex it will throw an error if the NFS share is disconnected.
All I'm saying is this is going to require setup on your part and isn't an "ideal" situation. It will probably be frustrating and you may just give up. 1 local pool with your jail is definitely easier and a better idea.