Pinion angle really has no bearing on an IRS. The housing is mounted solid to the frame so it doesn't move with the suspension. There may be some designed into the housing to frame mounts but this would be minimal and only for u-joint longevity.
"Rotating" the housing, as you say, for pinion angle will have no bearing on half-shaft or wheel movement as this is controlled by the suspension arms. You could actually have the pinion pointing straight up and the half shafts wouldn't care...
Mark