The Shirobako Specials were the adaptations of the anime inside of Shirobako. The second episode Third Aerial Girls Squad and The Magnificent Kotobuki are not only similar premise wise; cute girls flying in fighter jets, but the very same director who did Shirobako works on Kotobuki. Basically Third Aerial Girls Squad come to real life. Whether this was completely intentional or not, it appears to be a subtle nod at Shirobako.
Just as Exodus! and 3rd Girls' Fighter Wing are the fictional series made by the cast of Shirobako, Kujibiki Unbalance is the fictional anime watched by the characters of Genshiken. Both OVAs were made directly after their main series to complement their on-disc release.