I am trying this too. I've not managed yet. But here's a list of things which DONT work.
Using ImageMagick 'import' to get a screen grab, this gives a black window.
Using X-Windows to throw the window to another computer which does support screencast. Again Black window.
Using VNC to send the window to another computer. Again black window.
So dont try these methods.
In the end I set up a camera on a tri-pod to record minecraft. Here are my results http://lazybrush.wordpress.com/2013/07/ ... pberry-pi/