Maybe somebody will correct me if I'm wrong, but I think you cannot put extroot on a raid device. It's because the mdadm process starts much later than mounting the root partition (see scripts in /etc/rc.d/).
Creating raid devices at every boot is easy, see the file /etc/config/mdadm. You should put the correct uuid in the config array section. You can find it with mdadm --detail /dev/md0.