I have nothing against trying new things; however it is best to try to experiment in a "controlled environment".
So for sake of argument, lets say you are able to get FreeNas to boot from the USB fine and it even sees all the hardware.
- Be careful that you don't accidentally select the OSX Drive to add to a vDev otherwise that drive is now toast (for the most part)
- If you are talking about USB attached storage, then that is most definitely not a good idea
- External drives for the most part is not a good idea unless they are via proper methods (Never a USB Hub.. ;) )
- If you get this working, then decide that you need to boot up OSX guess what happens to that file share that everyone is using?
- Guess they will have to wait until you reboot back into FreeNas
- I have idea what OSX may/will do to either the USB or the external drive when it is up and running; hopefully nothing that would change/modify either but who knows?
Since I don't own a MAC I can only suggest that if you are experimenting then perhaps you could see if there is a way to actually remove/replace the existing hard drive (shelving the current one to prevent data loss). From my understanding it this system was just gathering dust anyways.
All that being said, I would say that this should be considered as an experiment at best and never considered a "Production" worthy endeavor. People tend to get upset when their data goes bye-bye. :)