OK, here's a suggestion. It took me a little longer than planned to get round to this because of general Christmas hectic-ness, I'm sure you understand :) This is a pretty simple process that only requires you to do a couple of things:
1) Create a HTML snippet
Log in to your sc_admin and add an HTML snippet element to your contact form fields. Insert the code below into it:
<div class="lbl"><label for="promocode">Promotion Code</label></div>
<div class="obj"><input type="text" name="promocode" id="promocode" maxlength="30" value="" /></div>
This will create a "fake" field in your form. This field is not recognised by the form processor, but we can use it to attach a bit of JavaScript validation to.
2) JavaScript validation
Open sc_admin/js/forms.js
Inside the validateContact() function you'll find a line that looks like this:
if ($("p.err").length > 0) { return false; }
Add the following code before that line:
if ($("#promocode").val() != 'freeoffer2010') {
$("#promocode").addClass("err").after("<p class=\"err\"><em>Please complete the promotion code</em></p>");
}
So unless the user types "freeoffer2010" into that field it will return an error.