I'm also going to be cosplaying as Malik at Anime Boston this year.
What I'm going to do, is have my arm just underneath the cloak. That way, I'll be preventing any discomfort when walking around. What I'm going to do for inside of his one-armed sleeve, is have a piece of styrofoam in the shape of the top of an arm. Just for the lulz. And I'm going to have a string attached to it so I can have it lift up and down.
His jacket is long enough to hide a full-length arm, so that's what I'll be doing and my arm is going to be under a piece of fabric so it won't accidentally show up in any photos...