Is it possible to "lock down" a course from 8-3, for example, and prevent "students" from accessing until after 3?
Not easily. You'd have to keep changing the dates of the course start, each day at 8am to lock them out, and have it set so students can't access a course before the start date (which is a checkbox on the settings tab).
Every day at 8am, you'd set the course start day/time to be 3pm that same day.
Why do you want to do this? That might help guide possible solutions. What do you want students to be able to do while they are locked out?
Retrieving data ...