Here's a very dirty hack that will get the get parameters into the form by adding in:
[[$_GET['subject']]]
into the appropriate input field.
<div class="[+subject.errorClass+][+subject.requiredClass+]">
<label for="subject">Subject</label>
<input class="wide" type="text" name="subject" id="subject" placeholder="Subject" value="[+subject.value+][[$_GET['subject']]]">
</div>
Any changes on the form will override this at submission.