I already tried the Foreground class node from Xith3d, however it seems that it does nothing
but moving the attached geometry to position (0,0,0).
It renderes the attached geometry RELITIVE to 0,0,0, you can still add a TransformGroup to move it around -- if you couldn't it would be pretty useless.
Don't forget about the three different camera modes either (View.VIEW_NORMAL, View.VIEW_FIXED_POSITION, View.VIEW_FIXED)
You can disable the Z-buffer on ANY object simply by setting an attribute:
Appearance a = new Appearance();
The Background and Foreground nodes do not do this for you.
Checkout my SkyBox code which combines a Background Node and disabling the Z-buffer: http://xith.org/showsrc.php?src=src/org/xith3d/geometry/SkyBox.java
I found that with my HUD it is better NOT to disable the Z-buffer as that has the side-effect of not playing nicely with alpha layers. However, if you don't care about that then disabling the Z-buffer on the Foreground node should work.