Is it possible ...

[TooMuchData]

... to attach an unformatted hard drive directly to a jail?

 

[anodos]

... to attach an unformatted hard drive directly to a jail?

Not practical. You could probably create some devfs rules on the FreeNAS host to make the raw device available inside the jail, but this would (1) not be persistent across reboots, (2) might seriously undermine the security of your jail, and (3) has a decent chance of exploding in your face in interesting ways.

The better question is: why would anyone want to do this?

 

[Ericloewe]

might seriously undermine the security of your jail

I think this is the important point. Not necessarily as a security risk, but because jails were designed to prevent this, so you're swimming against the current.