I've decided that I don't like the lack of forward weaponry on the larger Imperial ships, and here's my house rule to change it. there is no change to Dorsal, Keel, or Prow mounts.
Port: Fires to Port. Half (round up) of Port mounts may be designated to fire Fore and Port, but only if the mount designated does not hold weapons with Broadside in the weapon component's name.
Starboard: Fires to Starboard. Half (round up) of Starboard mounts may be designated to fire Fore and Starboard, but only if the mount designated does not hold weapons with Broadside in the weapon component's name.
This means that a Dauntless mounting non-Broadside weapons in its one Port mount and one Starboard mount can bring them both to bear Fore in addition to its Prow mount. A Lunar mounting non-Broadside weapons in one of its Port mounts and one of its Starboard mounts can bring them to bear Fore in addition to its Prow mount, and it can still fill the remaining one Port mount and one Starboard mount with Broadside weapons.
This upguns the larger ships considerably and makes them much more flexible in combat, especially allowing some forward-firing capability in Cruisers mounting an Armoured Prow. I know this doesn't match up with Battlefleet Gothic, and I don't really care.