Using a "work around" some people have figured out how to have full control over the Aperture with the Canon 5D mkII. Well, it works! ISO and shutter speed automatically adjust to the right settings...
You can also lock those down too if so desired using the AELock
1) Use manual mode or Av mode and set your aperture and ISO
2) Press and hold the DOF preview button and then press the lens release button and twist then lens 1-2mm until the live view turns off.
3) Turn on live view.
4) Press record - shutter speed should adjust and the ISO should stay at your desired setting. You can also lock the shutter speed and ISO by using the AELock, so if for some reason your ISO jumps up, lock it down using a bright object (light). The Aperture will read as 00 but it will record what you manually set it as....
This may break camera, who knows... Don't blame me!