I think what was being suggested is close to what you are doing but differs a little. Rather than using Security Assist to create the rule to check for the duplicate user you should be adding a server side validation with CSS Form Builder or Validation Toolkits server side validations.
The idea is that you have a recordset that is filtered on the entered value for the user, then with the server side validation you are checking that recordset to see if there are any rows in it. The recordset will only have rows in it if there are matches in the table. So this is why you apply the server validation for a number type and use the total rows of the recordset as the number to validate.
What this will do in the end is if the user enters a value that already exists this recordset will try to find a match, if one is found there will be 1 row in the recordset. Next the server validations will check the total rows to make sure that it is not greater than 0, if it is the validation will fail. You can then have your validation show if around the error message you want to display to the user to let them know that the value is already in use.
