I have a little problem with the z buffer when displaying 2d graphics. Look by yourself :
My custom scene node renders the white 2d graphics and the blue 3d object. How to be sure that the 2d graphics will appear in front of all the other objects ?
Is the only solution to render the 2d graphics at last ?
The simplest way is indeed to render the 2D last. Another option would be to make your 2D things 3D. (So, use 3d rectangles with textures and put them closer to the camera. But the first solution is much simpler.