checking if username exists in real-time for account sign up

Another approach is to provide another button for checking the availabilty.  Then you can do the work in and iframe or pop up.

Ajax is the ONLY way to do this without a full form submit request, page refresh response in the browser.

Unless you would like to replace the HTTP request/response browser application with something like Flash/Flex/Air user interface or similar technology.

But the normal way to do what you would like to do is to use AJAX which is not as complex as many make it sound.  With CF7 there are several comunity grown AJAX frame works, CFAJAX http://www.indiankey.com/cfajax/ and ajaxCFC http://ajaxcfc.riaforge.org/ are two of the best known.

Or you can simply role your own using the xmlhttprequest javascript object http://www.w3schools.com/dom/dom_http.asp and simple DHTML javascript to modify the DOM document object model.