Setting class-weights in a batch in where a certain class is not present
I don't think that the weight you use matters, whether you set it to $10^6$ or $10^8$. This is because, as examples of that class are not in the batch, the loss function for that batch will not have any of those examples contributing, so that weight won't appear. If something does not appear in the loss function, it is not used at all. For that reason, it doesn't really matter what weight to use.