If you're that concerned, set up a 3 way mirror. You could make the 3rd member an external device, connect it, do the proper commands to attach it to the 2 way mirror, let it resilver, then detach it from the mirror. By doing that you would have an exact copy (not a bit for bit copy, but more of a file system copy) on an external device. You could then
lose one of the 2 or heck even both of the other devices in the mirror and still have it.
"zfs special device" makes no sense to me.
zfs metadata is all part of the zfs "filesystem" structure.
zfs metadata without the underlying blocks is not very useful, it would be like having all the UFS inodes and not having the data in the inodes.
zfs send | zfs receive to send snapshots to external devices is the standard way of backing up zfs datasets. You can probably do a recursive snapshot and get all of them.
If you do zpool status you see what vdevs make up your mirror, it's either a partition or an entire device, so you could always dd just a partition, but there is no guarantee that it would actually restore correctly on a new device.