Well first of all the texture you are talking about isn't a property of the camera, but nevertheless it's possible. It will be a sprite or a quad parented to the camera, the you will have to mess up with entityorder (look in the documentation) if you want to ensure that it draws itself over everything else.
You'll also need to use CameraClsMode to disable colour buffer clearing on the HUD camera. Otherwise all you'll see is your HUD, and everything else black.