The Pihole software is able to run on inside a virtual machine on a supported guest operating system (e.g. Debian) on a FreeBsd host (like FreeNas). This is route is based on virutualization. Once a FreeBsd port of the Pihole software is available Pihole can run inside a Iocage jail on e.g. FreeNas. Iocage jails are the default application container on FreeBsd, another application container is Docker. The major advantage of running Pihole inside an application container instead in a virtual machine is that application containerization has a smaller memory and CPU footprint.
I prefer application containerization over virtualization because of less resource requirements. My FreeNas host is under dimmensioned in memory. Please consider porting Pihole to FreeBsd.
The FreeNas plugin feature is a way of adding functionality to FreeNas in a user frendly way (no command line involved). Providing a FreeNas plugin for installing and configuring (e.g. adding mount points to the jail) would be the ultimate integration of Pihole in FreeNas. For me this would be a nice to have feature, the FreeBsd port together with a small howto install and configure Pihole on FreeBsd would be the minimum requirement.