Most RPT jets in Oz can takeoff with 300 meters runway visual range, provided the crew has had the approved training. I suppose the reason why 300meters is the limit is due to the engine failure on the take off run scenario. Sudden failure of an engine will induce some swing to the side of the runway no matter how sharp the pilot is. It is very easy to "lose" the center line in this case with such low vis. Crews are trained in the simulator to handle this sort of problem. I think that takeoffs in vis below 300meters can be made if head up displays are used that give the crew an artificial runway enviroment.
One other point, how do the fire crews find the aircraft in the fog?