Headaches.

Ok, so this is going to get quite complicated.

I'm trying to replicate the control seen on the video link on the last post.
This is getting the motion capture rig (Mo) to drive Locators (L). Those Locators then drive the Skinned Rig (Skin). The Locators would be held in Groups (G) that are driven by a separate IK rig (IK).
Even at this point it doesn't work - as the locators are driven in world space, and thus their groups have absolutely no control as physical parents.
It gets worse.
Ik rig must also be driven by Mo.
This, however I seem to package it (as the images describe), creates an infinite loop.
At best I was able to make it 'work' - though this drove the locators in an additive fashion - making them rotate and translate twice as much as they should.

There were also other solutions - though these created dual dependencies and wouldn't allow the IK to be keyed.

I could quite easily build an FK setup - as I could:
Control Curve (CC) drives Mo. G is physical parent to CC. G is grouped within Mo rig.
This carries the Control, while still allowing the MotionCapture to continue - though it breaks up the control rig to sitting inside the MotionCapture rig.
This means that this way there can't be a possibility of IK - as it needs an unbroken hierarchy.

As you can see in the notes below, I started thinking about baking keys.
Something I would later realise is that this would relieve the attributes of dual dependencies.

Another error i continued making (as I was testing all of this solely on the hips to toes) was to use parent constraints to all of the drivers - even though most of the controls don't translate - they only need to rotate within the hierarchy.

The last note shows my final breakthrough and trial (which was late at night up to the eyeballs with energy drinks).