I guess the three big considerations with storage are starting hardware cost, maintenace costs/lifetime, and return of energy in v. energy recovered.
I am no great fan of batteries -- for those three reasons -- so I enjoy creative ideas along your lines.
The nearest existing model that comes to my mind is elevated water storage. It is pumped uphill to storage during surplus times, and allowed to return downhill through a turbine to recover the energy. I have heard quotes of 90 to 95% efficency in those models. There is a huge one at Ludington, Michigan. Water is pumped up hill from a Nuke at night (low use time on the grid) and recovered during the day.
I think I recall someone on here that actually pumped water uphill on their own property using purchased grid electricity -- in Virginia seems to come to mind. Anyway, they had an offset time billing so electricity was cheaper at night for them. They would recover it during the day, when electricity cost more. By gaming the ulitity, they were actually making money.
I am guessing you are thinking more mechanically? You will have to look out for high entry costs, and mechanical losses. I would think we could reasonably design a mechanical system with very low maintenance, and very long lifetiem.
But if you would like to brainstorm it a bit, I am game.
