You can use ensnare. Move the unit you want slowed to a secluded area and have it attack some units with 9999 life and 0 dmg. ALso have a queen with ensnare nearby and enough energy to cast it(which is controled by the computer who also commands the 9999 life units) The queen will most likely ensnare your unit as long as you run a zerg AI script, and that is the only spell available, so no parasite or broodlings. Test it, I'm not sure this will work, but it might.