I have to say that I would probably not even let the "hoarding" begin. When he puts more cars on the couch than he can physically play with at one time, I would either be getting him to pick some out to play with or finding a way to slide the others back into the general population, LoL. Our rule is if you put it down, you are done. The toy is fair game, now! (Some exceptions, of course)
I think you are right, just keep doing what you are doing. It's a tough concept to grasp, because of the age group. But it can be done!!
One thing I wanted to ask (respectfully), is why people say that toddlers "can't" share? But that they can take turns? I don't understand. Is that not the same basic idea, but using different terminology? Either way we are teaching that you don't always get to play with everything you want to, immediately when you want. Either way, it's the same social skill and the same principle, no? I understand the differences that Reggio pointed out earlier, but doesn't it just come down to semantics?