whatever you decide to make your halo out of, be it geometry, paint effects or what you can use a parent constraint and the halo will follow the position and orientation of your character's head. Or if you just want position you can just use a point constraint. Select the object you want the halo to follow first, and then the halo itself second and apply the constraint. You can do this before or after the motion capture is applied, it doesn't matter. The easiest way is to position the halo where you want it, then open the constraint option box and choose Maintain Offset. Apply the constraint and it will always stay a relative distance from the object its constrained to.