Not sure if this would matter, but when you build the lightmassimportance volume are you on CSG 'Add' or 'Subtract'? Because if you are on subtract, it may do the inverse of what you want. I would make sure you are on CSG ADD when you build the lightmass importance volume, then try re-baking.
I'm using the catmul-clark subDs yes, but the zbrush workflow is building a bunch of boolean objects in modo, addition and subtraction. Converting my mesh to dynamesh in Zbrush and start subtracting and adding pieces. It lets you add holes and cut shapes a lot easier than having to model it all as once piece.
Subtractive Meshes: Enable Tool: Display Options: Double to see backfaces or Insert the Mesh as an additive, move it into place, and then use Tool: Polygroup: Group as Dynamesh Sub to turn it into a subtractive volume. As for why they are displayed inverted, this is intended behavior to make it easy to tell if it's going…