If I can resurrect this old thread to make a couple of points from an American standpoint, even though I've probably said this before somewhere or other here.
I can't conceive of notifying the police or other law enforcement agency before setting out on a nude hike, either alone or as a group. As far as I know, it really isn't legal anywhere in the U.S., unless you count taking a long walk on a legal nude beach, of which there are a couple. For a nudist club that has the space, and I know of at least one that does, it naturally wouldn't be a problem. I don't know of any nudist club that has a farm, though.
The problem is, understanding what the law actually says in any given place can be difficult. Legal codes continually change over the years for all kinds of reasons and I doubt any lawmaker had nudity in mind when the first codes were assembled, from Hammurabi on down to today. Often as not, each specific law was written in response to something. If something isn't a problem, then the law probably won't "speak to it." But the problem is in the eye of the beholder, isn't it?
Not all laws, rules and regulations are written in committee of the legislature and passed by deliberative bodies. But some give authority to various agencies to essentially write their own rules, which they do. Again, the rules are written to address certain situations. The National Park Service, for instance, in the regulations for each so-called national seashore (where you just might like to be nude) have paragraph after paragraph about vehicles on the beach but nothing about nudity. Is that good?
Maybe, maybe not. If there are no signs specifically prohibiting nudity, does that make it legal? Not hardly, as we say, since the absence of a sign doesn't make everything legal. So, let's say you decide to contact the park manager and see what they say. You ask, "Is it legal to be nude on the beach at Shipwreck Point?" The answer you get might be, "Yes, but it soon will be illegal." In other words, bringing up the question raises their consciousness on the issue. Besides, you might not be the only person asking the question and the other people might not like seeing naked old men on their favorite spot on the beach.
So the solution is a heavy dose of discretion, which is good advice in any situation.