Painting NavMesh area's on terrain

I have been experimenting recently with the NavMesh and agents for my RTS game. As neat and awesome that it is, the NavMesh feature seems to be biased toward games where the navigation is object based and not so much terrain. And I say this because you cannot define areas in on your terrain since there are no objects to specify as terrain.

I have looked at the NavMesh Components library but it requires that you add objects all over the place that you want the baking to consider, which is extremely tedious and impracticable for a larger terrain map.

The feature I see lacking is the ability to paint or assign areas of a baked nav mesh that are not objects and I am curious if there is a way to do this?

The main reason I ask is because I want to be able to make the navigation across rough terrain less desirable than say a road or grassy field. I also want to dynamically change the areas based on the assigned use. Such as a grassy field becoming farm land so obviously shouldn't be walked through.

If this is not possible, what are some other solutions that people have used to create their own 'nav mesh' per say. At this time it seems that creating my own navigation system will be required that allows me to paint out areas on top of my terrain.

Any and all suggestions and guidance is appreciated.