Ultimately, scheduling shifts is going to be both easier to manage and more cost efficient than what you're suggesting. Uber has a good idea of how many drivers they need at any given time. This ideal distribution of supply will never align perfectly with the supply pool's natural scheduling preferences. You can try to use economic incentives to force these two distributions to overlap, or you can just exert control (that you rightfully have over employees) and force your employees to work when you need them.