You can't easily select that unless the stuff you want is on a dedicated relatively low traffic hostname. If everything is served off e.g. ads.baidu.cn then you have to decrypt all ad traffic, which is a lot.
You can select a random subset very easily at the layer-3/4 level. It's really not that different from just adding a host behind a layer-4 load balancer.
...and actually it doesn't have to be completely random. You could select specific IP addresses to intercept.
