Hello Patrick,
regarding screen overlay, we have seen the link below for implementation of screen overlay on javascript side.
http://analyticalgraphicsinc.github.io/cesium-google-earth-examples/examples/screenOverlay.html
but is it possible for us to handle the same in CZML format cause in our application we are rendering everything through CZML.
if so could you please provide us the sample CZML doing the same.
thanks and regards,
chandrika