Why not a vacuume?
Just evacuate your entire robot. Its less dense then helium or hydrogen.
Or better yet, why not raise the pressure in the building where the regionals are at. That way, if your frame is sealed, the air in your robot is less dense then the air in the stadium.
When you weigh, be sure to use a dead battery (remember electrons have mass)
If you're really adventurous, you could short the terminals on your battery and boil off some electrolyte.
Also, try to cut some weight out of the RC, works every time.
In addition be sure to sabatoge the scale. When the scale is zeroed, put somthing heavy on it, like a battery. Then the scale is "zeroed" at about 13.5 lbs.
Oh by the way, don't actually do any of this stuff, its not GP
Good luck yall!
-Andy