This is actually how the new version of phpScheduleIt is being built. You can set up just the times that are available and the rest are not displayed. I should have an early version of the app available for feedback soon.
Nick, I wonder what happened to this "hand picking" idea for available times? I'm a new user trying out pre-release 2.1, and am coming up against exactly the problem that my availability is different almost every day of the year, and I can't see how to make phpScheduleIt manage this.