I was asked last October by some all terrain fan friends, to derive a route to a given place in Portugal only on gravel roads.
I immediately thought OSM to perform this task, so I filtered highways information (track, path, unclassified, etc). Unfortunately it was not possible to calculate a direct route without hitting tarmac roads, so I used QGis to create some fancy maps (21 maps), where I could show my friends all the roads hitting a 5km radius around a direct line from point A and B. As I cannot grant correctness of information provided by OSM, that would be a task to perform while driving to the desired place.
Here is a preview of the maps created during this task: