I think stock fanmounts in the side panels would be counter-intuitive to the HAF in the HAF XB
Keep in mind that the case is designed with terrific front => back airflow. The top fanmount already messes with that. Side mounts would completely negate it and it wouldn't be a "HAF" XB anymore.
Don't get me wrong, it's a great option for watercoolers and i would've bought that extra panel the day it was released. Just think that CM releases enough cases that the HAF series needs to stay focused on air-flow/direction.
I'm currently trying to plan out an idea to completely remove the mobo support rails and make a mobo tray out of acrylic that hugs my mobo a little more and frees up some more room on the sides. Right now it's just an idea and dunno how much i'll bring to reality, which is why i'm not typing this in my build log right now