uhm not sure if you ment to say what you are saying. 100 feet would mean they were doing a cat II approach which is unavailable for that runway. now if you are refering to continueing past the mda (you can do this only if you have the runway environment in sight) then yes they could descend another 100 feet. If you have the vis before you get to the faf then you can shoot the approach. if the vis drops after you pass the faf you can continue the approach, if the vis continues to drop but you have the runway environment (approach lights, reils, threshold, etc, etc,) you can land. rvr, ceilings, vis, mean nothing once you pass the faf. from there on out you are going to go have a look no matter what happens to the vis. and even if you got down there and the rvr was reporting 600, but you could see the runway environment you can land. and your last sentence was uncalled for.