It's still not the same. Overlay planes are a completely separate pool of video memory (different drawing surface) from the main plane. You can scribble all over them and not disturb the contents of the main plane. It's a common hi-end feature available in the hardware and the native OpenGL that is not available through jogl.
Hi, I have seen application using underlay / overlay functionality on Solaris system with very low price/quality cards but as GKW mentionned you can do the same with FBO and I am not sure that performance will be decrease with it... Did you give it a try?