That's true about the e21 bolsters, I hadn't considered that. It's mainly the seat bottom I was thinking of, though. The 9C3 - from what I've seen - has a very flat bottom, probably because it's a police car seat. (I expect duty belts and high bolsters don't mix.)
Unfortunately, vinyl doesn't appear to be an option for Corbeau seats: unless Simeon is right, and they do offer custom coverings, it's cloth, leather, and suede only. The 9C3 seats are a light blue/gray cloth, almost certainly dingy after many years of neglect.
Thanks for the input, all. If I find a solution or pull the trigger on anything, I'll post an update.