“It wont be quite so easy as there has to be a dozen different models of the 400. I think the latest is 747-400XQLR. Each aircraft has a different base weight and capacity weight that comes into play.”
But, IIRC, all 747-400s have the engines mounted exactly at the same distance from the centerline, for starters. Daily observation, as was previously pointed out, could nail down VR for a particular scheduled flight.
BTW, on the blog there are links to a 747 takeoff cockpit Youtube there is a great head-on Youtube of a 747 during takeoff shot straight down the runway from a public access location. A terrorist with only an RC airframe might even achieve a direct hit from such a vantage point where only TWO dimensions are involved!
Runway security may become a higher priority than it is today.
If we are to become paranoid about what someone can do then we must lock down everything and close all roadways. I could easily just bump you at highway speed from the road and maybe kill you. I could knock you over as you road a bicycle. We could even just push you down a flight of stairs.
There are no protections in the real world from all evil. It is a simple fact of life.