Interesting problem and use of IsoVist. I think they way you have it set up right now is just to find the best spot for a single camera. What you really want is to be able to add several cameras.
I started some work on it, but the problem is the region Union fails sometimes. But it might be a simple way to get there.
Yes, the Boolean Union is super slow and also fails easily. Maybe there is a better way, but you should be able to get pretty close using that method. Once it works, just leave it running over night.