CiviVolunteer: is there a way to send mailings to smart groups targeting shifts for volunteers
The "who signed up for a particular shifts" part is the problem right now. At present, you could use scheduled reminders to send an email to anyone with any volunteer activity between X and Y o'clock, but if you have two shifts (or projects, for that matter) you won't have a way to distinguish between them, so they'll all get the same message.

We're expecting to finish a proper integration with scheduled reminders sometime before March 2017, but probably not in 2016.