The rules are the same for all National Parks. I am not aware of any Letterboxes in NPs but of course am not an expert. There are a number of Letterboxes on Mt. Desert Island but not, to the best of my knowledge, on park land.

And hidden by the Rangers may make a difference. I will not defend the thinking on that one - just state that done by staff they may feel they have more control.

Acadia's cache is an Earthcache.