There is now hidden field of course, but you have two options,
1. Use the Text based Templates instead of the XSL modify ITEM.HTM OR ITEM.TXT and take out the [QUESTION], and [ANSWER] tokens.
2. Use the autoresponder like you stated put a textbox on the form with a default value with a hidden label. In the Class field for the control define a class line txtHidden and put it in your portal.css display:none use this field for the Autoresponder email
Steve