I am Andreas.
I have found a intresting thing for using RAID with setup storage.
Also a RAID 1 with one drive is allowed with setupstorage.
disk_config disk1 bootable:1 primary - 50 - - logical - 800 - - logical - 3000- - - disk_config raid raid1 /boot disk1.1 ext2 rw,errors=remount-ro raid1 swap disk1.5 swap sw raid1 / disk1.6 ext3 rw
After installation you can expand it with:
mdadm --grow -n 2
to a real raid. Then you must rebuild it like one disk has failed.