Britax has been promising anti rebound bars for their Clicktight models in the US for a while now, but I hadn't seen them for sale anywhere. I checked the Britax site today, and they are now available! They work for the Marathon CT, Boulevard CT, and Advocate CT models.

http://www.britaxusa.com/accessories/anti-rebound-bar-clicktight-convertibles

Not sure how long they've been around, but I figure that I can't be the only one who missed this the first time around. Figured I'd share