There is no reward for doing "Mad King Says" more than once per day. I miss the reward system from GW1, where you could show up every time, if you wanted to, and receive bags. It was fun. Also, I miss rock-paper-scissors.
There isn't much to do during Halloween anyway, once you have done the new achievements (more of that, please! ) and the dailies. Plus, farming the Labyrinth, Ascent to Madness, or the JP gets boring quickly, and none of those are particularly social activities (as there is no time, really, to talk to other people during those activities and have actual fun together).