That's no biggie. You can include a warning on the form that would be normally hidden by the same mechanism that hid the input field.

   Title:
   [_________________________]

   Text:
   [_________________________]
   [_________________________]
   [_________________________]
   [_________________________]
   [_________________________]

   LEAVE EMPTY!! [_]            <- "subject" field.
                                   Normally hidden by CSS.
                                   Only non-CSS clients
                                   and overriding CSS
                                   clients will see.
[download]

And when you receive a form with the field set, you could republish the form (pre-populated) to the client, asking him to resubmit it with the field empty.