As I said, you might be able to use a foot-candle meter held directly on the screen to give you an approximation of the actual foot-Lamberts. I'm afraid there is no equal in photography to the simple Radio Shack sound level meter.
Foot-candle meters are used in motion picture and video production for setting up lighting so that the light ratios remain within the desired production values. That is why you can find meters that read out in foot-candles.
Measurement instruments that read out in foot-Lamberts are laboratory level test instruments not needed for general photographic uses.
What I can do, is try out my PhotoResearch LiteMate III foot-candle meter with my rear projection Pioneer Elite and see if it appoximates the values given by Pioneer for screen brightness.