The hull can be part of the room it does not need to be a big box around the whole map (that is a bad idea when it comes to vis I think ... not to sure but it seems like it would be). Think of your map as a building you are makeing in outer space. If you have a gap in it where air can leak out into the void of space you are gonna choke or vis will choke in this case. So use caulk brushes to seal off any holes that air could escape from and then texture the side of the brush that can be seen (if any). That is what you were asking about when you were talking about hulls right?
I put the new in b.