The South Coast Route, also known as the South Coast Wilderness Trail is a concrete example of a mostly pathless hiking route:
A general description of the hiking conditions along the route:
Hiking the Olympic Coast is unique in that the routes for the most part follow the natural land features found along the shore. Overland trails aid hikers in navigating sections of the coast that are not feasible or safe to hike at shore level. Certain sections of coastal routes are also only passable at mid-to-low tides; these sections should be identified and planned out prior to a trip.
While walking this route you will mostly be on beaches and intertidal rocky shores. These sections have no visible path. Sections where walking directly along the shore is not possible, or too dangerous, are bypassed by sections of visible path. A route relation including only the visible path sections would be missing large sections of the route.
A video tour for those interested in more detail:
Mapping this as a route relation with untagged member ways representing the pathless shore sections seems perfectly reasonable to me. The only problem is that some mappers will see this as an error and add highway=path to the untagged ways. not:highway=path to combat this may work, but I feel that a positive tag indicating a pathless route section would be even clearer.