SC2Mapster

I would do it via a Create Persistent effect that orders the units to the point unless there is an enemy within a certain range of a buff on the unit. The Train and Arm Magazine type abilities or the Spawn type behaviour can be given requirements limiting the number of each spawned in a mothership requirement like fashion and automatically spawn units. I would use the train effect to apply a buff that uses an Issue Order effect to tell them to hold position when not moving.