I think if you will attach GI data like irradiance cashe or photon map to object per vertex (or any other way that provide in time object deformation) instead of just object/world space mapping. It will help to make fast and good GI with motion blur. With this first/GI passes can be rendered in time between shutter open and close. And then final pass will load baked GI data and render image with full GI and Motion blur. To do this photon hider must support RiShuter call with same open/close time.