part (b) requires the permission of the copyright owner. Not permission from the owner of a copy.
True. But since you aren't selling a derivative work, you don't need permission. The idea is this: Once the customer has the exact copy, you sell them your own, unique patch that does not violate any copyright laws. Essentially making 2 separate sales that accomplish the same thing.
I think this could be a way around the issue, but only in a very dry reading of the law. It's pretty clear what the spirit of this law is, despite the loopholes in the letter. As such, I doubt the courts would let such a defense succeed.