Not millions, with HD sculpting it can have billions. No polygon modeller can handle this. The best way is to bake it in ZBrush.

I have no idea of the features of ZB4. So I cannot tell you if they make more UV-options. But this never has been a deal breaker for the big studios. Most gamedev studios create a lowpoly mesh in a polygon modeler with uv-maps and transfer it to ZBrush for sculpting.
And this works perfectly now with GoZ.

But to be honest, I dont see your problem. You can make good uv-maps in Blender or in 3d-Coat. You dont need this feature badly.

Why they do not support free tools? The reason is very simple: They deal like a commercial company. If most of their (paying and registered) customers are using Maya, then they start with Maya. If the SDK of Maya, Modo and C4D is easy to support then they will do this first and more applications will follow (they said, that there will be more apps later. But I expect that Max has more priority than Blender).