Show unique values already in use?

This is a great module! My suggestion is to have already used values to show up as read-only and/or greyed out.
I'm using this to store unique codes for each user and it would be nice to be able to see what codes are already in use.
So I could select the code to use from a select list, but already used codes will be greyed out and read only so that I can't select it. Or perhaps have some autocomplete suggestions as you type.

Thanks