If someone can see nude bodies thru the gap in a weathered fence, I'd think they have to have their face right up to the boards peering in. And in that case, if there are any complaints, I'd agree saying that their are "peeping tom" laws against such actions.
We had this same issue at our previous home. I'd tried for a few years to get the neighbors to go in half to build a block wall, to match the others we'd already built on our own dime. They refused. The boards dried and shrunk . I was out back, like I normally was, and our neighbor (woman), that had seen me several times before, suddenly says loudly, "oh my god, enough!"
Two days later, her husband's at our front door asking me if I still want to put up the block wall! We did but she still found a way to look over the wall and see me a few times. After we moved away and had been gone for about a year, we got a Christmas card with a note inside. The note read something like, "we wish you were still our neighbors despite our difference in attire in our backyards. We miss what good neighbors you really were." Apparently the new neighbors were not as good as the ones that used to run around naked all the time!
The pros and cons of spending the money to put up more wooden slats to cover those caps ... might be better than irritating your neighbors and having them cause a stink. Or ... talk to them, tell them that you've lived this way ever since you moved in and the fence gave you privacy and you're wondering what your opinions are. Maybe they'll surprise you and say they don't care.
My fence did the same thing too. How frustrating. I knew there would be some shrinking eventually so I planted Jasmine in front of some of the most visible areas.
I had sugar snaps growing up on one side and pole beans in another area. Made for great summer snacking in the yard. I also had an area I cultivated real well and dumped in handfuls of sunflower seeds from my bird feeder and it looked lush and green all summer along the fence.
Find out what grows well in the light along your fence and become a gardener.
It will give you some peace of mind and the satisfaction of growing. A friend just told me he grows hops. Think I'll try it this year.
Do your neighbors know what you do on the other side of the fence? Maybe you need to just tell them. You have taken all the legal precautions by building the fence, the rest is on them!
Only 6ft tall is all you need? I would need a 10ft fence to be able to be nude in the back yard.
There are trees along the back fence that help in the summer time but the neighbors on both sides can see over the top of our 6ft fence from their patios.
I will look into the privacy screen to put at the top of the fences.