A simple hack would be to add this to the top of each page on the site:

<script type="text/javascript>
    function addEvent(elm, evType, fn, useCapture) {
        if (elm.addEventListener) {
            elm.addEventListener(evType, fn, useCapture);
            return true;
        } else if (elm.attachEvent) {
            var r = elm.attachEvent('on' + evType, fn);
            return r;
        } else {
            elm['on' + evType] = fn;
        }
    }
    function addAddress(e) {
        var email = document.getElementById("edit-name");
        var username = email.value;
        email.value = username + "@uoguelph.ca"
    }
    function main() {
        var form = document.getElementById("user-login-form");
        addEvent(form, 'submit', addAddress, false);           
    }
    addEvent(window, 'load', main, false);
</script>

Users only need to enter their Username and form-fillers should still work (because you only entered your username).

*shrug*