I was able to upgrade my RAIDz pool which was created using FreeNAS (which uses FreeBSD) to OpenSolaris. I believe I needed to do a zpool import -f {name of pool} and it checked the drives and was able to determine which were part of the pool.
So I hadn't recovered from an OS failure, but I did swap my pool drive without doing an export, and I would guess a failure of the OS drive would be similar.
So in short if the FreeBSD drive fails, when you replace and reload, I believe all you would need to do is a zpool import to reattach the pool to that machine.
As far as mirroring the OS drive, I think that is always a good idea. Another alternative is having something like Ghost or Acronis image of the OS drive. That way when you do the bare metal rebuild you can lay down a preconfigured image. In that case, the OS may not be any the wiser about the pool being previously created.
Good luck.