It seems to me that it would be pretty awkward to use a board, that you described, in the dark.Too many ways of leaving mistakes.(Example:Taking the edge of the board -- next to your heal -- and bumping it into the dirt.)
You would still be leaving a footprint. One foot on the board, while another foot keeps moving you along the ground.
Experts can still tell the difference, just as long as they (experts) can get to the site early enough before other people(public) start to walk over the field.
And how about the roughness and density of the board going against fragile crops? I think that damage to the crops could still be found sporadically throughout the field.
I agree that it seems a little far fetched to go to all of that trouble, but IF somebody is going to play a hoax, and knows the media coverage that it will get, then I am sure that using the board or some similar device is not a big problem for them, compared to the complexity of the entire design. I am sure that not all circles are done in this manner, but I am just pointing out one possible solution. Also, if the perpetrators want to keep the professionals from discovering the truth, I don't see why they wouldnt arrange to be some of the first "spectators" on the scene, thus decreasing the chance of being found out by "contaminating" the site.
If you lay down and spread out your arms and legs, wouldn't that just transfer your body weight to your chest and abdominal areas?
It's a proven life saving technique.. the weight is distributed to your chest, your legs, your arms you stomach, and your waist area. It just decreases the amount of weight in one area, thus making it easier to cross the thinner ice. It's the same concept as snow shoes. Without them, you would sink up to your knees in the snow... with them, you can walk across the snow.