Hey, I know this advice might be a lit late but I imagine you would achieve this with render layers. I’m not even sure if Unreal 4 supports customizing render layers. I have not looked into that yet but it is how you would do it in other engines.
As an example you may render a motion blur layer that only has your blurring objects, it would do another render pass with non-blurred objects then layer the two. Usually the same principle used for things that glow, the glow effect is usually just another render layer.
Just keep in mind that with each render layer you take a performance hit. Hope this points you in the right direction, even though it is several months late.
On a similar note you can override the default global motion blur settings on each individual camera. This can be adjusted and controlled via blueprints but it is a camera option not object based. Object based things would still be a new render layer.