Could it be possible to modify the custom formations widget so that it works with unload commands? The idea is to be able to drop transportable turrets in formation (the stock area unload command is fine for mobile units but it's not well suited for transportable turrets).
I've been trying to do it myself but since I don't really know lua it's been more of a trial and error endeavor.
Alternatively, a widget which simply tells transport VTOLs to unload exactly where they are would also work (you could load the transportable turrets, order the transport VTOLs to move using custom formations and then just order the transports to put the turrets down right below themselves).