Haven't looked into it, but an obvious problem would be, that there is no convention about covers in cbz's. The actual cover is just the alphabetical first image in the archive. So there is no robust way to check if that is a cover or just the first side, making batch conversion difficult. And then there is the question as to how to name the cover to make shure, it is the first image.
A possibility would be to compare calibres cover with the first image and only set it, if they are different.
I may take a look at this, but won't promise anything, as this would be not the most robust feature ever
