Use JavaScript in your template to add the onchange handler:
<script type="text/javascript">
window.onload = function()
{
document.getElementById("id_yourfield").onchange = function()
{
doStuff();
};
};
</script>
If you're using a JavaScript library, it will no doubt have a more
succinct way to do this which registers the event handler properly
instead of assigning a function to an event property.
Jonathan.