i still think that navigation map should be generated at runtime (probabilistic roadmap comes into mind), so you dont have to use any additional data, libraries or anything. To do that would be needed:
1. When a grid is loaded, generate N random samples of terrain using vmaps near creature spawn zones (aggro radius?), that samples are made at floor level.
2. For each of those points, find the K nearest points and test if they're in line of sight. If they are, connect them into the graph
3. when a creature moves, calculate shortest path to target using the calculated graph, if the average distance between a straight line and the path is above a treshold, move the creature by the path, otherwise move in straignt line
well, just an idea.