And finally just mounted the thing under /opt (& updated /etc/fstab). (I could of course have used LVM here too, but frankly in this case I didn't see any point in that, and I've already wasted enough time trying to get this working...)
So now the RAID partition is ready to use, and I've got plenty of disk space. :-)
Update: there are still some issues with this RAID device of mine. Upon reboot, it fails to mount even though I have it in fstab, and sometimes (after reboot) it appears to be in an inactive state and cannot be mounted even manually. See the follow-up question I posted."