Mind you that on a dual channel approach at 400 feet the aircraft will trim nose up. Reverting to manual flight to attempt a landing after this point can be quite a challenge and you are better of going around even when visual.
On a single channel approach you dont have this problem, but single channel autopilot minimum use height is 158ft AGL so still above your 100ft RA. Although its a bit academic you would need to fly the last 58ft to your minimums manually, which is not prohibited. However my outfit does not allow manually flown CATII approaches.
Anyway a system failure on a CATII or III approach will most of the time lead to a go-around.