There are people who have done precisely that in places like South Korea where there is very overt discrimination against educated women returning to the work force after their children are a little older. If I were starting my own firm that's exactly what I'd do (and some day, I might).

There are some places in New York that at least have the right idea (I don't know about the implementation): http://www.cravath.com/benefits ("We have an associate re-entry program that is designed to foster a connection between the Firm and those associates who leave the Firm for primarily family-related reasons... we hope that these former associates will consider returning to the Firm if they decide to practice law again.").