I imagine I should be able to use what @midgunner66 posted above to compensate for the camera motion at this point. The motion blur shader appears to compensate for camera movement across the entire screen, so perhaps adding the camera’s velocity to the first person object’s material might be enough?
EDIT: Though it would be nice if the shader could automatically compensate for certain flagged attached-to-camera objects instead.
EDIT2: Alternatively, I could investigate if there’s a way to render out first-person details (hands/accessories) on a separate layer or render target, and then comp that on top of the previous render layer.