Accidentally found this one, but very pleased....
Its a little hard to explain. I was re-visiting my tachometer and speedometer gages. I cound not get the rotate sprite to work (its in the documentation, but I'm guessing its not until the next release or I am jus not getting it

).
Next I tried to make the 3d model follow the player camera. This would have worked well, except the FOV changes according to vehicle speed and makes the gages jump and in size on the screen.
Anyways, since I couldn't the the gages needles to rotate as sprites or orientation following the camera, I have a 3d model of the gage. I am additing a additional viewport thru script to focus on the model. In the past, I always made a background object black and then hid everything under a hill, in a building or behind grandstands).
I found out very much by accident, if you move everything way below the surface and do not have a skybox or anything else visible beyond the needle, it becomes transparent.
Other types of games could use the same theory too, such a make your current weapon rotate in 3d, etc.
Here is the picture, probably explains better than my text

