Ummm... the skull doesn't extend to muliplayer and Xbox live.
What you need is an oddball, a plasma pistol, and no secondary weapon.
1: Walk up to the oddball while you plasma pistol has an overcarge bolt glowing.
2: Even though you cannot see the icon. Press X to pick up the ball, BUT! As you do, walk backwards! It's important, that you DO NOT release the trigger and let the gun fire.
If done correctly, you will drop the oddball to the ground along with the plasma pistol, and your targeting reticle and weapons will be gone.
To eliminate the Rest of the HUD. You need to cut as much out as possible. Turn off shields, and the motion sensor.
Then using a video editing program, apply letterbox bars to hide the score, and motion sensor, then add a thinner one to the top (how you'd do this, I have no idea, but it's how it works on the machinemas I've seen.), and voila.
Your HUD is hidden, but be ready to practice with your cameraman skills to get the shot you want with some of the screen blacked out after the editing process.