They absolutely can fake it. And yes, putting them away and stopping work is reward. Most people feed them when they put them away too. Double bonus. It takes a very smart horse and just the right situation to learn this IMO. Something has to connect in their very simple brains that the lameness is connected to not working and most horses do not think like that.
I knew a mule once who would limp on every leg on the way away fro the barn to not have to work. There was nothing wrong with him. He tried it every time. If one foot didnt work, he'd try another. People had fallen for it so many times and put him away that he forever tried to fake a limp and not have to work. He was sound as a dollar on the way home and in his field. There is no question he was very smart and figured it out clearly from actually being lame at one time or another.
Again, its very very rare, but it absolutely happens. IMO, anytime a horse shows lameness, you believe it until you can prove otherwise and a vet backs you up like we did with that mule after we saw a clear pattern in his behavior. If you ride a horse that really has a real lameness, even intermittent, you could seriously damage it. Always assume the animal really IS lame until you can prove otherwise with vet backup.