I really want to convince myself that the Carrack remote turret is no a bug, but a feature, so I checked HardPoint. There are a host of remote turrets there that fit the chin mount of the Vanguard, including one that mounts a pair of S4 guns.
There's also one there that mounts a pair of S7 guns and fits the Carrack's S5 weapons hardpoint. Now that would make a nice remote turret for the chin of the Polaris! How much cash would you pay to buy a remote turret that fits a pair of S7 weapons onto an S5 hardpoint?