I managed to get this working as follows. Instead of getting the weapon socket location each frame (in world space), I just get it once from the CDO (in weapon space). Since the socket location relative to the root never changes, this is exactly the offset that’s needed. In the Two-Bone IK node, I set it to use BCS_BoneSpace, set the effector bone to the right-hand weapon bone, and set the effector location (offset) to the variable holding the weapon-space offset. That offset is then applied in bone space to IK the left hand exactly where I want. Hopefully it will be useful for someone else.