Stephen,
My knee-jerk reaction is that a reflective device would be too prone to error.
I measure some courses, and many time calibrate, during darker periods when cars have lights on. If I passed in front of one of the newer cars with really bright headlights, I can imagine false readings. Or, sunlight reflecting off of a shiny surface in a concentrated beam.
I, for one, would not trust a reflective unit.
Just my thoughts.