What does Special:ListUsers =/= list of all users with SUL mean?
I thought the problem was to find account names that are not being used. If you search for a name you think you might want to use it returns a list of existing accounts.
What is "SUL"?
http://en.wikipedia.org/wiki/Wikipedia:Unified_login
You can't use name if this one is unified login, even if this name not exist in local project. Special:ListUsers is only a local list. Probably most SULs exist in en Wikpedia, but not all.
przykuta