Coordinates to draw a Heads-Up Display

I want to set up my simulation so that whenever I click on an agent I get a display on
the screen of a 3D visualization of the agent's brain (a neural network controller in my case).
However, I want the visualization to always appear in front of the camera on a certain
area of the screen, no matter what the camera is actually looking at (it currently follows
the main agent wherever it goes). Is there any way to specify coordinates relative to the camera?
This would sort of be like a Heads-Up Display common in games. It's also similar to the placement
of text messages on certain areas of the screen. I basically want to use the monitor's coordinate plane
instead of the one in the simulation, but I still want to be able to draw 3D shapes.

-Jacob