The default can be set using a simple jQuery script:
<script type="text/javascript" src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript">
// set no conflict mode for jquery
var $jQ = jQuery.noConflict();
var pproduct $jQ.getQueryString({ ID: "product" });
var productLength = $("pproduct").val().length;
$(document).ready(function () {
if (productLength >= 6) {
$('#product').attr(“value”, pproduct);
});
</script>
I neither tested nor refined that script.
Please ensure your developers fully test it before moving to production.