How do I render shadow-only pass from Indigo/Luxrender?

I am wanting to composite some 3D objects over background video, but I don’t know how to only render a shadow-only pass so that I can composite it over the background plate. I want to use either Indigo or Lux because of their unbiased nature and ability to use HDRI lightprobes. So does anybody know how to do this? Thanks in advance!