Thanks, all. I'm new here, and there's a lot of information to digest.

Pieren, thanks for the tips on tracktype and surface. I will use them.

About "highway=service". That tag is actually already in existence for some of the non-paved "roads" inside the park -- I think from an import of Mass. state government data. These are all roads that are actually used by official vehicles (for access to reservoir pumping stations and the like). But, perhaps these should just be tagged as "highway=track, tracktype=grade1, motorcar=no"

For "motorcar=no", I meant that the road's surface was such that a regular car COULD use it, if allowed. However, park rules are such that the public is not ALLOWED to use any of the roads/trails. That appears to be what the wiki says the motorcar tag means: "Access permission for cars.".