Get the UUID from a mostly dead hard drive?
Have you tried looking at the kernel log messages?
If, as you said earlier, it tried to root from the drive at least once, you can try to find it in the /var/log/system.log, for example:

$ grep 'rooting\ via' /var/log/system.log
Oct 24 18:01:44 localhost kernel[0]: rooting via boot-uuid from /chosen: 4AB3D289-884F-379C-AF7B-************
Oct 25 11:21:57 localhost kernel[0]: rooting via boot-uuid from /chosen: 4AB3D289-884F-379C-AF7B-************
Oct 28 10:30:54 localhost kernel[0]: rooting via boot-uuid from /chosen: 4AB3D289-884F-379C-AF7B-************
...


Those logs are usually kept for months, so you should be able to get it.