Over the past few months I've been working on just this idea.
I have created a map on wich all legal tracks and paths are highlighted.Since I live in the Netherlands, its focused on that area (Belgium, The Netherlands and Luxembourgh). I have also added a routing engine wich prefers dirtroads, so making a route is really easy. The route can be exported as a GPX file. Have a look on http://www.allroadmap.nl
With this same idea I have created a garmin-map, wich installs directly in Mapsource (Belgium, The Netherlands and Luxembourgh only). You can have a look at: http://data.allroadmap.nl/8jul2011.exe
It would be really easy to expand the site to include other area's of the world.