I would be interested in seeing an example of that setup if you wouldn't mind uploading? I think my setup doesn't work well, at least in that blend file I uploaded. You might be able to get infinite wrist twist using Transformation constraint and extrapolate while keying with euler, doubt it will work rotating past -180 or…
Hows this, twist bone parented to hand with IK chain setup to forearm bone. Copy rotation getting local Y value from hand at 0.5 and hand and twist bone set to Euler YXZ to evaluate Y first https://dl.dropboxusercontent.com/u/1693140/Arm_multi_bone.blend (Initial rig/model is not mine, found it on blend swap or a forum…