i think you can specify to the calendar popup the initial position to popup at, so you can make it always appear within the modal
Yes, I already use this to reposition the calendar in case it is located at the bottom of the screen.
However, the modal panel might be too small to display the whole calendar, so I need the calendar to overflow.
Set trimOverlayedElements="false" for rich:modalPanel.
That worked great for IE, but FF still trims the calendar. I'll try to combine them both, overflow: visible in FF and setTrimOverlayedElements(false) for IE.