There's no W coordinate in Maya unfortunately Have you considered baking the information to textures and then either compiling it back into the model in Unity or just reading it direct from the textures?
That's weird. I don't recall ever experiencing such a thing. If you can, make a small example file w/ the problem. At the least there could be a valid bug report in there.
export the lowest level and import the highest level as a layer, works :) wish it would be that easy with zbrush files, w ould love to interact more between both