I always thought that image based motion blur should work as a post effect. Somehow I feel confused, though, because I can’t get motion blur to work on an image sequence. It works ok if I render an image in my animation and run it through the vector blur node (all in one go), but if I try rendering an image using the sequence editor it won’t work anymore. So I can’t create motion blur on a beforehand rendered image sequence. Which I always thought was the actual point of having image based motion blur for.
No, there’s no possible way of Blender knowing which way the object is moving, and how fast, unless it knows the velocity of the object, so that’s why you can’t do it in the sequencer.
However, it is possible do it in nodes with a sequence of pre-rendered images, if you also rendered the ‘speed’ as a separate image when you initially rendered it - this shouldn’t take too much time, methinks? :-?