2D Motion Blur should work on clouds. 2D (post processed) motion blur on clouds can have some problems because the clouds are semi-transparent entities with many overlapping points that can potentially move at different speeds or even different directions, but in many cases the results are acceptable.
Your scene looks like it should be able to render OK with 2D Motion Blur, but clearly it's not, so I think there's a bug here. I'm investigating now.
Matt