I see one possible loophole we can use. Under gun emplacement it reads that the type of weapon depends on the model itself. It then gives a few suggestsions on shooting models. To me, the stats of a gun emplacement are in the hands of the players, so why not take advantage of that fact? I don't see anything preventing it in the rules. I fully expect to see Ork Supa Mega Dakka Quads, Necron Gauss AA, and Tau Markerlight towers, why not Tyranids, too?
Why not make some sort of Tyranid "gun" emplacement? Maybe a long-barreled Impaler Cannon with Skyfire. Then add in the special rule "Synaptic Uplink: Only Tyranids may use this weapon. No, the don't use their opposeable thumbless hands, they brain-jack in" Of course, it's useless in a torny, but for normal games, it should be ok.
Edit: Also, the Skyfire result on the Mysterious Objective rule could help too if Lady Luck is smiling. A full brood of Hive Guard getting Skyfire would be a nasty surprise for Flyers.
Edit 2: Or if the Implare AA Cannon won't jive, why not take the Quad-Qun entry and then tac on a modified special rule. Broken Sensors: The Quad-Gun's tracking systems are damaged and can not tell friend from foe. Any unit that get's close may regester their side as friendlies at the gun's bio-scanner. At the end of each player's Shooting Phase, the Quad-Gun auto-targets the nearest of that Flyers or Flying Monster at BS 3. If one player has a model in 3 inches, his units are exempt. If both players are within 3 inches, the gun's sensors regester both as friendlies and the gun shuts down.
This rule is more impartial and adds the additional goal of needing to rush the gun emplacement which could add some fun the game.