Even better, open VehicleSize.txt and change the number of engiens a base ship can have.
The "Number of Engines per move" setting DOES work now, and I've altered my tech set to require two engines per move on battle cruisers, three per move on battleships and dreadnoughts, and 5 per move on baseships. Since the engine limit on a base ship is 15 and an engine bonus of 1 or better will give the required minimum movement of 4 with this number, it works well enough and I do see AI built baseships in my games.