Remember that officers share the stats of the unit they are attached to.

And this is a very good thing, as it could be used to include the underground maps, if there's a battle in this roads the map for the battle would be a simple cave/tunel, catcomb, dwarf palace mine or sthg skaven or lizardmen (settlement and so).
You must understand the limitations of modding. The maps are generated from the campaign map, so if you put a road between mountains then the map will be a road between mountains. You can add special maps to areas using the custom tiles method, however the custom tile is limited to the playable area so if you do use this method the distant objects will still be generated from the campaign map. It might be possible to make a three or so underground maps and repeat them along the road.

However, if it's like RTW, black roads are impossible unless you sacrifice a road type to tunnels, which is a bit much to ask. However, you can place 3D objects on the map (there are a couple of different ways of doing this) to add fake mountains to cover the valley, hence creating a tunnel effect.

I think it could be done, but not as well as some of you I'm sure hope, and it would be a lot of work.

I think if the team are going to do something like this, it's definately on their "To do if we have time" list.