What happens if you were to include the following form.
<form action="" method="post" name="site_search_form" id="site_search_form" onSubmit="document.location='http://www.google.com/search?q=site%3A'+window.location.host+' '+this.q.value; return false">
Search this site:
<input name="q" type="text" id="q" size="80">
<input type="submit" value="Search" name="search">
</form>
The above is purely a theory and has not been tried by me.