Bungiex88,
I'm in the same boat as you. A little digging on the forum has led me to the conclusion that what we want is not recommended or supported. Reasons include best practices, reliability, and setting a standard that ensures TrueNAS will perform well for the majority. Google
freenas partition boot drive for some reading on why.
All that said, as an engineer who understands FreeBSD I can see the driving factors and am willing to take responsibility for my own actions. I can also fix it when something horrible happens. If you still want to pursue this and are willing to not blame anyone else when your server does something bad (read, please don't come here and write something negative about TrueNAS after modifying it outside of support scope), then there is a
forum post with instructions. It is a bit dated and will require some understanding of FreeBSD to making it work.
Best of luck,
Mike