Thanks a lot! Works like a charm. Even no issues with multiplayer setup or custom mesh without IK rig (just normal hand bones).
Edit: Just in case anybody else is trying to do this with two bone IK: After playing a bit with this setup I found out that looking upwards causes the left hand ellbow to look a bit strange. That means you have to add the joint target location as well. BoneSpace lowerarm_l or upperarm_l works both if you move the joint target arrows in the preview of the anim_bp somewhere near the left hand ellbow.