We have eaten at the Disney resorts many times without staying there. Never had to pay for parking. No way. Just tell the guard that you have a PS for the resturant and he gives you a pass.
I'm sure all sorts of people around Orlando have learned this and Disney encourages it because its good buisness.
By the way, you do not have to go to or through the magic kingdom parking to get to the Poly. Someone already said this, but I confirm it.
The disney guards have gotten much better post 9-11. In the old days, I hate to confess this. I use to park at the poly or contemporary instead of paying for parking and walk to the magic kingdom. I think it was just an old trick that many did, too many I guess because now they watch for it. In those days the guard just waved at you and you drove through and you waved back. Ah yes, those were the good ol days.
There still are some tricks, but I would never tell them over the internet.