To the best of my knowledge you are not allowed to add extra guns to an MA map (other than shore batteries) and I haven't heard anyone say this is going to be different in the next version. HTC are very strict about what can be put in an MA terrain. They can't afford to have the MA crash due to some object-related bug, it has to run 24/7 and they might not be there to reset the arena if it does. Essentially you can put in the air and V fields, towns, the strat facilities, tank town, ports and fleets, spawn points, shore batteries and that is it.
So my advice is that if you are thinking of doing or putting anything into your terrain that you have not already seen done in an existing MA terrain, PM Skuzzy before doing so. It is less frustrating to be told "no" before you spend a lot of time on it.