Many years ago, we built a RAID volume using DLT tape-drives, for a project we were working on. It was a huge pain, far more trouble than it was worth, and if just one tape went bad, ANYTHING written to the others was unreadable.
FWIW, this was about 10 years ago, so hardware has advanced a good bit. If you can find a RAID controller that will let you do it, enjoy. I wouldn't, though...one scratch, and your whole set of DVD's is useless.