"too overt and the message gets quickly lost when security kicks you out"

True, but also be prepared to be searched. If security can't legally search people, given the size of the event, they will very likely have police to patrol and do random checks "in case of terrorism", so hide your HK shirts, signs etc. because they would almost surely be confiscated for "inciting violence". If Blizzard fears any negative demonstration, they'll probably use every means for painting those supporting HK protests as violent disruptors and have police arresting them.