At the moment I have only been able to get the behaviour I want by copying the IK controller, align it to the FK direction, orient constrain the copy to the FK wrist with offset and then use the rotations on the copy to match the actual IK wrist to. This includes creating and removing a constraint though, which I'd really like not to, so what I'm looking for is a way to apply the same math as the constraint does without the need of the constraint.
By making the constraint as previously described I have found the constraint offset to be: ( -8.637529, -98.637351, -89.999763 ) and when the FKwrist is rotated (-89.99994, 1.184596, 0) the IKcon is: (-0.000178598, -88.81544, 0.000174945) As pivots are a bit askew these values doesn't map directly onto each other so I guess I'd need a conversion matrix of some sort as well but I'm not quite sure which and I'm beginning to doubt that this is the way to do it? Have I missed something completely or is it just a very annoying rig to match?
Btw. it's the Norman rig if anyone would be interested in that information.