It is correct that OSMAND can navigate where you specify a number of waypoints in a sequence and it can navigate along the path of a gpx-file. I have not seen the navigation coorporate with Mapillary to e.g. navigate to non mapped areas.
As far as I know there is no automatic help to find or navigate non mapped areas.
But one could run through OSM data from an area and compare it to sequences on Mapillarys and make waypoints or gpxpoints on paths that are not within a certain distance form a Mapillary image. Making a good optimized route is probably not easy, but it should be doable.