Animating the camera's focus during a render with motion blur From another perspective: "Focus tracking during exposure"

OK, so this is probably pretty obscure, but it looks like the motion blur system in cycles isn't taking into account camera focus over time.

[EDIT] Just to clarify, I'm using motion blur to refer to the renderer integrating over time. I'm not asking about an image space selective blur or approximation, but a physically accurate render and effect.

I haven't had any luck with either setting a moving object for focus tracking or setting keyframes in the distance field. In both cases focus does change correctly when moving between each frame, just not in a render with motion blur.




Attempting to focus track a moving cube (with a static one for reference): 


Is there something I'm doing wrong or is this on the todo list?

[EDIT]
In regard to what this might be used for:


Case 1. You actually want to integrate objects transitioning out of/in to focus such as in this photo: Fireworks refocused quickly during an explosion

There may be none or little actual movement for the motion blur but changing focus during an exposure can add interesting effects. Here, I'm use the name motion blur to refer to integrating over time which is what the effect actually does, rather than literal blur from motion.
Case 2. You have a shallow depth of field (and still want the effect that gives), but you want motion blur of a subject moving perpendicular to the focal plane without the defocusing effects of Case 1. Tracking the moving object (with focus only and the camera remaining stationary) would give this effect. In doing so the background will not be blurry from motion, but may change focus a little.
Motion blur is separate from Depth of field (camera blur). The object will only be more in focus if either the motion blur is turned down or there is less movement of the object relative to the camera.