I had the same problem. To get around this currently, I do a render without any volumetrics. Then I do a second render and select all of the objects in the scene and add them to the 'Render Selected' list, then invert the list to exclude those objects. This effectively makes those objects 'matte' (or 'holdout').
For a 2k image, I am averaging about 15 minutes a frame for my volume pass, with all of the individual light selects of the volume for complete control.