[Grim Tuesday]

Tie a ton (not literally) of balloons to your robot before you get it weighed.

Last time I saw the 'no lighter than air gasses' rule was 2010 so I suppose it would be legal.

However, it wouldn't help as the dictionary definition of weight "A body's relative mass or the quantity of matter contained by it, giving rise to a downward force; the heaviness of a person or thing." would not change no matter how much buoyant force you had lifting it. The apparent weight would; the actual weight wouldn't.