To be able to use the boot pool/fs for storage, create new zfs filesystems in it, etc. (probably under a big disclaimer saying that it’s not recommended)
No. Let me be extra super clear here, so there's no ambiguity.
At one point, the way old FreeNAS 0.7 used that sort of strategy (and the current iteration of that previous product, NAS4Free, still does). Try using that.
iX bought the name FreeNAS five years ago, and the design goal of FreeNAS for the last five years has been to create a storage appliance, similar to a Dell EqualLogic or EMC Storcenter. The point is to have a large pile of disks that can be upgraded/replaced/modified/etc without all the drama and complication of having to tiptoe around an OS installation sitting on the data disks.
The devs are creating FreeNAS as a way to test TrueNAS, their dedicated storage appliance that they sell as a competitive product in the software defined storage NAS marketplace. To do that, the ability to treat storage in this abstracted manner is a *requirement*.
Trying to support the OS on the data disks is a fail, because then you get into all the issues of "what happens when the OS isn't viable" or "what happens when the user insists on non-redundant storage for the OS" or "how do you handle the user complaining that there's 16GB missing from each data disk" or any of the other obvious problems from that strategy. It's a frickin' disaster of complexity and ambiguity as to unforeseen problems and issues and other ${mess}.
There is not going to be any interest by the devs in supporting this, because it goes 180' against one of the first design decisions they made, which was to keep the OS and data disks separate.
There's no interest in making FreeNAS work on every possible platform, because there's enough work involved just making FreeNAS work on the narrowly scoped set of hardware that is targeted as a supported environment.
So with all that said, please feel free to go try NAS4Free, which, being targeted at home users, probably still allows the OS to be on the data drives.