Machine isn't running right now, but I think it was bought in 2000 and probably hasn't been upgraded since around 2005. That's when Mandrake became Mandriva and the upgrade was known to be problematical, so it was never done.

I wonder if there is a live distro out there with a sufficiently recent kernel, and raid support that I could use to do the trick above? If I understand correctly, once I've gotten the right values into the superblocks, rebuilt the array and resynched, I should be able to boot up on my old kernel and have things still work.

Then I can look into upgrading to a newer kernel.