You could add a jQuery script to trigger a mouseup event when the user triggers a mousedown event on the draggable area. You have to add this in the head of the html for the modal windows:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:wicket="http://wicket.apache.org">
<head>
<script src="js/jquery-1.8.3.js"></script>
<script type="text/javascript">
$(function()
{
$(".w_caption").mousedown(function(){$(this).trigger("mouseup")});
});
</script>
</head>
<body>
.................
Make sure you also reference jQuery in the head of the modal html. (tested with jquery-1.8.3)