The *most* efficient way would be to use option 2 (GUITexture with alpha across the screen) for a single frame, setting the camera to not clear out the display buffer and then disable rendering of everything else in the scene. If you need the pause menu to be interactive, you would copy the screen to a render texture and use that texture as the background instead (this would require Unity Pro).
This is most likely overkill, just putting a simple GUITexture over the screen should run just fine.
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