this might be too late for you, since it looks like you have finished rigging and skinning.
i looked at your blocking ->
and if you want really really nice poses - it would do your character good if you added a joint between the shoulder and elbow, and a joint between the elbow and the wrist. This joint should be free transformed to create more fluid poses, so you would avoid those not so nice looking straight arms.
your set up (x shoulder, o elbow, C hand)
awesome posing setup (x shoulder, z awesome joint, o elbow, z awesome joint, C hand)
good luck. the idea is funny.