To be honest I heavily disagree. I've been using ZFS for quite a long time now (one year (the last) on FreeBSD and earlier on quite a few more on Solaris) and I've always used the approach where the main pool also provided the main (root) filesystem. Heck; even Sun did so themselves back in the days and they invented the whole thing in the first place.
There is nothing wrong with using the main pool as your root file system. Basically; if you don't then technically speaking you're wasting one of the available file systems in the first place, why would you? It doesn't make it hard to backup (it's just like any other regular filesystem out there; the only difference is that it was automatically created at the moment you created your pool). Sending it to other systems also doesn't cause further problems because it's quite easy to import a filesystem on a different location. In my opinion it's no different than using dump to make a snapshot from your (UFS) root filesystem, then using restore on another system to, well, restore it. It's not as if you're limited to overwriting your existing system.
Edit: In fact, not doing so only makes your live a whole lot harder which I think is absurd (no offense intended). Really; if I need to check var, and var only, I use zfs list zroot/var. If I need to check the status on my source filesystem I use zfs get readonly zroot/src. The more overhead you add, the more you need to type and the more easier it becomes to make mistakes. zfs get readonly zroot/root/src? That doesn't look like a very optimal situation to me; now I have to type root twice!
Never forget that us Unix administrators are lazy (or we should be). Having to type something out twice for no other reason than "the install script set this up" is, like I mentioned earlier, in my (strictly) personal opinion quite absurd.
(/edit)
Now, to answer the original question... It is possible without disk formatting, but you will need to do so using other media, like a rescue CD or such.
Also keep in mind that you probably need to edit loader.conf and rewrite the bootcode to keep the system bootable. Other than that I see no obstacles as to why this wouldn't work.
