well, kind of a combination, and I don't know how useful it would be, but you could use a Command object and select the DISTINCT of the datafield...which is basically DBlank's solution.
Taking kostya1122 idea in as well, in the comand object you could select a field as the first character of the field, and/or the first 2 characters to limit the list if they know the code that they are looking for.
Though this solution will remove the multiselect option (I believe, I don't use the parameter much).