Patrick... that's def. a good feature request -- but unfortunately QCalendar doesn't support that right now.
If you could, please go ahead and open up a ticket in the issue tracker for this... and better yet, if you're willing to try stuff out, you could look into implementing something like this, as well =)
It would obviously involve editing QCalendar.class.php as well as calendar.js... I'm not too sure, but I would assume that we would want to allow users to add a QChangeEvent action handler to QCalendar... in theory, qcodo would then render out the qcalendar control (which is actually just the IMG tag for the calendar icon) with a “onchange” on it. If so, then we would just need to add some code in calendar.js to go ahead and execute that “onchange” code in objControl.hideCalendar() in calendar.js
Hope that makes sense... let me know if you have any questions