MR had a brief article on this back in the dark ages (1960's I think). The key points that I recall were:
1) Four cables with spring clips connected to loops at each corner of the layout.
2) Each cable ran through its own set of pulleys, for each - one pulley directly over the respective layout corner, the other pulley over the common counterweight at (possibly within the stud space in) a nearby wall. Obviously the pulleys must be well secured across several structural ceiling joists.
3) Each of the four cables individually connected to the counter weight with a turnbuckle to permit adjusting the length.
4) The counterweight was a metal can filled with concrete with an embedded pipe handle at the top for the cable connections and was slightly lighter than the layout. I'd suggest an inverted U of pipe where you could add fitness weights on the verticle sides secured with a pipe end cap.
5) When the layout was in the stowed position at the ceiling, safety hooks at the ceiling prevented accidential lowering. I'd suggest one or more cable clamps placed on the cable between the spring clip and its overhead pulley to act as a stop so that scenery wouldn't be crushed against the ceiling. (Gee, why do all my trees have flat tops?)
6) To lower the layout, release the safety hooks, guide the layout down, place a sturdy board-shelf under the counterweight (on brackets) to support it in its elevated position, slide a separate saw horse under each end of the layout and disconnect the cable spring clips.
One would need to recognize that weight and potential energy of the layout and counterweight can pose a significant safety hazard which would require careful construction utilizing suitable materials and techniques.