While I was working on this problem, I came up with a very rough approximation that I'm now using to make up "UZR".
These are, of course, complete judgment calls. I welcome any ideas about how to refine this table. As far as a rough approximation, they seem reasonable to me, but I'm guessing that some improvements could be made.
If any roster makers see this, how do you guys convert major league defensive stats into ratings for your roster sets? Do you do something similar, but in reverse?