I'm not very familiar with this, but I think Ubuntu's desktop installer ("ubiquity") may not be able to handle RAID. I recall seeing RAID options in the "alternate" installer (or the server installer) but not the one that is used by the Live CD.
On the other hand, if the OS drive is not in the RAID array, you may be able to first install Koha Live into that drive, and then install the necessary packages to recognize and handle the RAID array. (e.g. dmraid)