The real question is even if the scales are there, how effective would they be. First how many people are going to stop even if they weigh themselves? I'm guess not many. How could it be accurate? A humid day is going to make it difficule somone who's cloths are saturated with water may not look like their weight has changed, or it might look like they've gained weight when they didn't.

The real focus should be on teaching people how to hydrate properly. Getting a handle on your sweat rates should be as important as your training when you're talking long distance racing. Making blanket statements is just dumb either way.