The Director uses the Nav File to generate infected when properly generated and analyzed. There is an attribute that is marked during the nav_analyze called "ESCAPE_ROUTE" this is the "line" from Start to Saferoom that the director uses as the path the Survivors must follow. I have been experimenting with how to overcome this using point templates to mix up the routes survivors must take and different nav files. Based on how the Director works it has been.... problematic? Just try to make as many intersting areas off to the sides of the main path as you can to hide goodies in L4D2 is promising to support functionality similar to this and it is not to far off.
Remember, Snake it around... Change elevation... You can make it appear there are multiple paths without actually compromising the flow of your map.



