What I meant was a feature that needs a function to validate. So the server validation will also look at the outcome of the function when validating the page.
The function would have an outcome of true or false (or any other values like "FAILED" "SUCCES" and would be called by the validation script of webassist.
In that case you can add more complex validations (like checking a database if the username or email exists) without breaking any of the webassist code.