What do the regulatory bodies say about landing in CAT IIIB?
The ability to perform autoland approaches requires specific approvals.  In the US, it is outlined in AC 120-28D Criteria for Approval of Category III Weather Minima for Takeoff, Landing, and Rollout. BTW, CAT I and CAT II approaches are covered by AC 120-29 Criteria for Approval of Category I and Category II Weather Minima for Approach.  

The AC describes the necessary installed equipment, specific crew training requirements, and the need for operational procedures.  Once approval is granted, the crew training and operational procedures become part of the airline Op Spec.  

If any hand flying is allowed, it will be spelled out in the Op Spec. Since Op Specs do vary from airline to airline, it's not possible to state definitively what is or is not allowed.  

That said, it is physically possible to manually take over and land the aircraft. If the crew has the runway in sight at 200 feet, it's really no different that a CAT I approach. At 50 feet on an autoland approach, the autopilot will be starting the flare and de-crab maneuver, so manual intervention at this point may or may not be allowable depending on the specific aircraft performance and the airline's operational philosophy.