I’ve always thought that there are a number of options around Malham Tarn, but never tried to overnight there. On busy days, plenty of vehicles park on the grass off the road and I don’t see why you couldn’t stop overnight. I’m sure you would leave nothing behind when you leave to show that you had been there.
There are also various places along the (very) minor road between Settle and Arncliffe (though not much East of Darnbrook House / Nab End. Its very open around this area.
Or you could go to The Station Inn at the Ribblehead viaduct. They allow camper vans overnight in their car park and I think it may be free if you eat a meal there.
I’m presuming that you are not looking for designated camp sites as there are a few of those in the area.
Good Luck.