I am currently working on a code plugin that enables pathfinding for a large number of units (even on large maps with dynamically changing environment).
I basically implemented the “Flow Field” approach from Supreme Commander 2 (plus some custom enhancements and modifications).
The plugin is not yet complete, but would anyone be interested in test-driving it?
If you are interested you can also tell me which feature you want to see in the final plugin and I will see if it can be added 
Currently working:
- initialize surface cost data from textures
- tile-based merging A* search
- flowfield generation
- steering an actor based on the flowfield
- dynamically moving target
- adding and removing actors for pathfinding
- visualize flowfield and generated paths
- Optionally create smoother flow fields by computing a sector ahead for each field
Still to do:
- Support multiple targets per agent (e.g. for zombies chasing more than one hero)
- Add multithreading support, so flow fields can be generated in a separate thread
- [Optional] Automatically generate surface data from navmeshes
- [Optional] Automatically generate surface data from geometry
Source: