Can I ask what time you were planning on going? I was allowed to park at the Poly last November, it was in the evening and we said we were headed to Trader Sam's. (Not a lie, genuinely what we did.) You don't make reservations for that bar, but it only opens in the evening. So during that time the security is more likely to let you in, but during the day time they must constantly battle against folks who try to avoid parking fees by stowing their cars at the MK resorts all day.
If you want to spend some time at the resort and its during the day time, it would be worth it to get a reservation at Kona Cafe for lunch (or breakfast if early). Then you are free to park. And they have great food that wont break the bank!