From what I’m guessing, there’s a difference between the actual POSTing action, and the return page.
So, the form posts, and then after all that is done, all that EE does is redirect you to a page. That redirected page does not get the POSTED variables to it as well. So you can’t use posted variables from that form.
What would seem to be necessary would be a very simple extension on the email_send (or whatever it’s called now) hook that stories the username in a cookie so you can use it on any further pages that you want to by that user.
On another note, if you’re trying to get posted variables, it’s much safer to use $_POST than $_REQUEST.