Other ways to validate quickform server side

Hi there, I have tried to post this before, there is functional limitations in terms of validating quickform server side. My issue is I need to register a custom rule which requires doing a lookup for duplicate usernames in the database. I handle get and post requests in different methods within the page class. This ideally means the way quickform validates by posting to itself is virtually impossible in this circumstance. I handle my get requests which executrs a class method say _doGet and post method _doPost. Now if I post how would it be possible to display the form again (I use DB_DataObject_Formbuilder and a Flexy quickform renderer) if a server side validation rule check has occured before actually allowing it to post ? Please let me know its one last hurdle. Currently to prevent having to find a server side validation solution, I am trying an ajax connection within the javascript rule function however gettijng it to return true and false within the ajax callback function is trikky right now.