But this isn't Garmin-specific. It's the choice of whether a way should be considered "routeable" or not. That's useful info for all the autoroute programs and GPS navigators.
The trouble with the "mkgmap:unpaved=???" approach is that it duplicates existing functionality in OSM. We should strive to get the existing functionality better specified if it doesn't already do the job for us. Otherwise, mapping effort will be spent on adding a set of tags to OSM which only benefit the Garmin routable maps project. What about the TomTom people? Or the AndNav2 users? They'll want to know about routeable or unrouteable unpaved roads too.The whole point of using a mkgmap: prefix is that it does not force a particular OSM tag to be used for a garmin gps specific purpose.