So, if you are flying to CAT IIIb minima (100m RVR) and you don't get the FMA mode changing from LOC to ROLLOUT at 5', can you continue to land?
You have no rollout guidance and no reversion to CAT IIIa minima.
Indeed, an interesting question.
Now, in the 'ole TriStar, there is no doubt (in the described scenario) you are going to land, but without rollout guidance, you could, I suppose, end up in the weeds.
In personally performing well over 800 autolands in the Lockheed three holer, never once has rollout not been annunciated (and provided), however, if the LOC (ground facility) trips off at the least opportune time...well, this would definitely not be good if the visibility was severely restricted.