It's finally here - a working Hover Turret! This NPC was cut from Portal 2 but its entity class survived. Now it has a model too. Demo map included. Installation instructions in the README file.
Another quick note: In the demo map, a phys_ballsocket is used in conjunction with a pair of keyframe_ropes. Technically, the correct entity is ent_hover_turret_tether, but there doesn't seem to be any way of finding out how to actually use that.