I think the only way to accomplish that is to either submit the form and have it run a check or you could use an AJAX query where say you set it to "on blur", it triggers a process to check the username against the database column. Writing the AJAX query could be a complex process... there are a couple of other extension developers that have a server object that will do just that. Check out the Spry Data Utilities extension from Linecraft: index.php
You will notice halfway down the page that they have an example of Spry Data Search that allows you to check a username "on the fly".
Hope that helps,