I should clarify a bit -- you could have a promoted property called "notify" or something, and the user can click a button that sets the field to the email address to send to -- then you submit the XML to a form library w/ a workflow on it...
Of course, that assumes you are using SharePoint? If not, then I really don't have any ideas...
Oh -- wait! There is an IP bug you could use to your advantage here -- in your rule that emails, switch views right before sending -- that always sends a blank email.....