I really don't believe your merkin story, but for the sake of discussion let's run with it. Look at the way our country's law enforcement has gone; people can be arrested for anything even if it isn't a crime, just because some whiny little bïtch complained or because the police didn't like that someone didn't cower and submit 100% to their authority. Within the last couple years there's been a few times where women were arrested for wearing a swimsuit, even a full sized one piece, to the swimming pool of their own apartment complex where they live, just because someone bïtched that it was "too revealing". I don't know whether those women had the charges thrown out or what, but even if they are it's an entire experience in public humiliation, wasted time, wasted money, stress, and frustration over absolutely nothing.
And then there was another time where two women (who were drunk) were walking naked down the street, and a cop rolled up and arrested them for violating the public nudity laws. They challenged the arrest in court because (1) where they lived it was perfectly legal for women to go topless just the same as men (yay equality!), and (2) the law specified that it was illegal to expose the genitals and the anus. Since they were standing upright, their buttocks alone were not illegal because their anuses were concealed. And in court, representing themselves, they asked the arresting cop (as a witness) one single question: "At any time did you see our genitals." (I'm paraphrasing that quote because I don't know how it was specifically worded.) Since they both had full pubic hair which concealed their genitals, which by definition would have been their actual labia/vulva, the cop under oath had to say, "No." That was their only question, and the judge threw out the case against them because they had not actually broken the law, thus effectively confirming that female nudity was legal as long as the female is not bent over and has enough pubic hair to conceal the genitals.
But that's a very rare sort of circumstance. In this country (the US) just about everywhere there are bullshit blanket laws that cover everything that someone might not like, in the form of "disorderly conduct" or "disturbing the peace" laws. All they do is give cops the leeway to arrest and charge anyone for anything that they or some whiny little bïtch complainer doesn't like. It would be well worth it to get a good lawyer to fight the charges because, after all, the court and law would have to define what constitutes disorderly conduct and orderly conduct and what makes the arrestable behavior "disorderly" or how it disturbs the peace, but that would be quite costly and most people don't have the extra money to pay for all that so they just accept the fine and probation and such. And sh!t like that can ruin a person's life over something that harms absolutely no one.
So to get back to your question, wearing nothing more than a merkin might possibly count as an acceptable covering (it isn't clothes) for the genitals to make that level of undress legal, or it may be viewed as not good enough and lead to your arrest anyway. It's illogical, irrational, and in my opinion unconstitutional that there are any laws that criminalize the nude body in any way, but that's just how things are for now and it will take a lot of brave people with a lot of resources to challenge the laws and hopefully get them repealed or suppressed by court precedent. If there's any truth to your story (again, I doubt it), you could talk to that guy who wears one of those things by that local lake and ask him about what he knows of the legality of it in that area. But why would you want to wear one? Just find places where you have the ability to go fully nude, whether a secluded spot in the woods, at home with a good sized property, a naturist resort or nude beach in the area, a regular beach after closing, or any federal land in the area (national forest/national park area, as it would be under federal jurisdiction and there are no federal laws against nudity, except where there are local park rules against nudity such as Yellowstone and other high-visitor-count areas). Years ago I went camping on the land of a national forest and was nude the entire time (except for a chilly morning when I put on sweats for an hour or so), and there's a state park beach near me where I love to go at sunset for skinnydipping because everybody else leaves at that time and I can freely go nude despite it not being legal here.
naturist since 1990