I don't claim to know a ton about this, but I did look into it when there was talk of James Rodgers getting a medical redshirt.
This was from OSU's website after the decision was made:
He qualified for a medical hardship per NCAA rules - 14.2.4 - "a student-athlete is eligibile if he/she has participated in a maximum of 30 percent of the scheduled games" - Rodgers participated in four of the team's first five games which is actually 33 percent, but the NCAA rounds up the total games to make it four competitions allowed - he missed the Oct. 2 Arizona State game with a concussion. The medical hardship was granted by the Pac-10 Faculty Athletic Representatives Committee.
If this is what it is based on, it doesn't look promising. Cory played in 5 games as a freshman, 5 games as a sophomore, 11 games as a junior, and six this year. All are over that 30% mark. Of course, I'm absolutely hoping I'm wrong.