BR had windows on the front lav upstairs on their old 747 combis. Once on a TPE-LAX flight where it was "daytime" but due to the time change the blinds in the cabin were all closed and most pax were sleeping, from my seat I observed that upon pax entering the lav, 'most' of the time shortly after the door closed the briliant sunlight of 40,000 feet came streaming through the cracks under and around the door. Shortly after the lav flushed, the sunlight would go away and the pax would depart the lav into the darkness of the sleeping cabin. Thus I surmized that the window in the lav was a very popular feature.