Was looking for some online articles that might have a good idea or two in how to approach this but not finding much. Most of it deals with the various ways to generate and follow a path to a known destination type.
Basically the idea is that you have an existing node graph and want to utilize that data in order to locate entrances into the current area an AI unit would want to defend. So think of an AI unit assigned a task of defending a point with a radius and wanting to choose the best direction to face while waiting for enemies to show up that would be pathing to the defend point.
The critical point that I was looking for input on would be how one might identify the entrance points. Or the nodes that are the doorways into the area being defended.