From memory, which is becoming more suspect by the year, indeed the stop end could be ignored, either u/s or below limits, on very long runways. The start end RVR can be assessed by PIREPS, counting the centreline lights. Some operators might use edge lights, giving a slight diagonal distance and building in a buffer, but the centreline lights must still be working if <400m. That means mid-point must be working. The pilot can not assess that.