I can't speak to the likelihood of success (don't really know anyone who's dealt with that with Carmax).
However no such thing as too far up. In fact, most success esp. for goodwills come when you contact the right person on up. There's always that couple people in the organization who see the bigger picture (keep you a happy customer) and have it in their power to waive stringent policies here and there to get things done. I say go for it...highlight your good history, how it's negatively hurting you, any mitigating factors, etc...
