I believe the Shadow Catcher plugin has to have shadows from a light to work. Radiosity shadows are different.
A light dome is a whole bunch lights, usually attached to a sphere. It surrounds the scene in lights, which can look a lot like a radiosity render illuminated by an image. This would be a good solution if you have an external scene, shadow catcher would work with it. If you have an indoor scene it probably would not work as nicely. You could always do a combination of light dome and radiosity, render times would suffer though.