Interestingly enough, I was working on something like this today. If you are keeping your OSX drive, you can install rEFInd and use that as a boot manager. Make sure you read the OSX warnings - that should allow you to boot FreeNAS from the USB drive. I'm not going to comment on whether or not this will be viable, or even recommended but you should be able to do it. I was trying to use an old Mac Mini as a FreeNAS device and use the internal drive as storage to serve files to Mac, Linux, and Android devices.