Re: WPAjax and responseText
I’m not sure if it will work though.
Would it be too insulting to refer you to the source code of the WPAjax class? There are answers there if you are able to interpret the code.
Also, if you are not actually returning some output at the page you are requesting, then it might be returning undefined. Again I’m not sure, being that I (like fel64) volunteer my time to answer queries here, I am not necessarily able to fully troubleshoot your issue (especially since you only present half of the equation – i.e. the client side AJAX request).
You catch more flies with honey. Etc., etc.