We’ve noticed that a Large Ball which is only inflated to 15.5" can be pushed under the Barrier if we ram it really hard. Is this a legal maneuver?
No, this is not legal. Any attempts to push or wedge an inflated Large Ball under the Barrier would be seen as unsafe and an attempt to damage the field, and thus a violation of <S1>, which is quoted below for your convenience.
Simply put, do not try and put Large Balls under the Barrier. The Barrier was designed so Robots and BuckyBalls could pass underneath, not Large Balls.