I would disagree.
FIRST has said that creating a mechanical device to hit the bar and lower your light is legal. I see this as being no different. A flag mounted atop a long, flexible wire would be fine.
I have gone back and forth with FIRST on this, in private, a number of times. While my question was never answered to my own satisfaction, they did make it clear to me that their intent was to prevent robots from physically attaching themselves to the midfield barrier such that efforts to detach them could cause damage to the field.
Mechanical devices that react off the bottom of the bar, that exert small amounts of force, and that do not engage the bar in such a way as to make removing it difficult would not be deemed illegal.
Or, at least, that is the final impression I was given. I did get 4 or 5 conflicting answers, so, who knows?