Microburst,
That's the same as saying, "If, after you pass the ABP, the reported vis goes below the minimum, can you land if you have visual reference at the DH" The answer is of course, yes you can.
In the same way, if the vis goes below minimum after ABP on a CatIIIb, you can land.
There are minimum RVR values, and then minimum visual reference criteria for each catagory of approach, except CatIIIb which basically has no visual reference criteria, you just let it land!!