Here's the thing: When IK is on, the rotations of the Goal are in worldspace. When IK is off, the rotations of the Goal are relative to the rotations, etc, of the next immediate body part downthe chain... Therefore,without knowing the rotations of every single body part in the chain, there is no real way to calculate the IK on rotations of the goal.

Remember, kids! Napalm is Nature's Toothpaste!