I do know that the water parks have refillable mugs, but I think you get a sticker that allows you to refill for one day, not your whole trip.
I think it would be really hard for the parks to have refillable mugs unless they add some pop stations away from the place where you order and pick up your food. All of the counter service places are set up for people to come into the line, order and then continue on to pick up their food. Unless they completely change things, I don't see how they could route people (plus you would need to hand your mug to someone behind the counter to refill - I'm sure that would break some food handling rules).